const string template2 = "Template2";
            const string template3 = "Template3";
            const string template1 = "template1";
            this.Nested(
                new Grid().Resources(
                    new ResourceDictionary
                        {
                            {
                                template1,
                                new TextBox().Text("t1").ToDataTemplate()
                            },
                            {
                                template2,
                                new Grid().RowDefinitions(
                                new RowDefinition().Height_Auto(),
                                new RowDefinition().Height_Auto()).Nested(
                                new TextBlock().Text("template2"),
                                new ContentPresenter().Grid_Row(1).Content(new Binding()).ContentTemplate(new Resource(template1))
                                ).ToDataTemplate()
                            },
                            {
                                template3,
                                new Grid().RowDefinitions(
                                new RowDefinition().Height_Auto(),
                                new RowDefinition().Height_Auto()).Nested(
                                new TextBlock().Text("template3"),
                                new ContentPresenter().Grid_Row(1).Content(new Binding()).ContentTemplate(new Resource(template2))
                                ).ToDataTemplate()
                            }
                        }).Nested(
                    new ContentPresenter().ContentTemplate(new Resource(template3))
                    )
                );

AP>            const string template2 = "Template2";
AP>            const string template3 = "Template3";
AP>            const string template1 = "template1";
AP>            this.Nested(
AP>                new Grid().Resources(
AP>                    new ResourceDictionary
AP>                        {
AP>                            {
AP>                                template1,
AP>                                new TextBox().Text("t1").ToDataTemplate()
AP>                            },
AP>                            {
AP>                                template2,
AP>                                new Grid().RowDefinitions(
AP>                                new RowDefinition().Height_Auto(),
AP>                                new RowDefinition().Height_Auto()).Nested(
AP>                                new TextBlock().Text("template2"),
AP>                                new ContentPresenter().Grid_Row(1).Content(new Binding()).ContentTemplate(new Resource(template1))
AP>                                ).ToDataTemplate()
AP>                            },
AP>                            {
AP>                                template3,
AP>                                new Grid().RowDefinitions(
AP>                                new RowDefinition().Height_Auto(),
AP>                                new RowDefinition().Height_Auto()).Nested(
AP>                                new TextBlock().Text("template3"),
AP>                                new ContentPresenter().Grid_Row(1).Content(new Binding()).ContentTemplate(new Resource(template2))
AP>                                ).ToDataTemplate()
AP>                            }
AP>                        }).Nested(
AP>                    new ContentPresenter().ContentTemplate(new Resource(template3))
AP>                    )
AP>                );
AP>

C>Думаю, в конце концов Вы придете к мысли, что не получится покрыть все сценарии, которые могут быть реализованы в XAML.
Что значит, не получится покрыть? На худой конец, можно просто реализовать паттерн билдер для построения XAML-а. И тогда любой код на XAML можно переписать на C#-е. Весь вопрос во временных затратах и в удобстве получаемого синтаксиса. Оценкой этих двух вещей я и занимаюсь сейчас, а Вы почему-то решили, что уже что-то сделано. Спасибо за помощь в этом процессе, ваши вопросы оказались весьма полезными .

<Page
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Page.Resources>
  <DataTemplate x:Key="FieldTemplate">
  <Grid>
  <Grid.ColumnDefinitions>
  <ColumnDefinition SharedSizeGroup="group"  Width="100"/>
   <ColumnDefinition Width="*"/>
  </Grid.ColumnDefinitions>
   <TextBlock  TextWrapping="Wrap"  Margin="4,2" VerticalAlignment="Center" Text="{Binding}"/>
  <TextBox Margin="2" VerticalAlignment="Center" Grid.Column="1"/>
  </Grid>
  </DataTemplate>
  <DataTemplate x:Key="DealerTemplate">
  <Grid Grid.IsSharedSizeScope="true">
  <Grid.RowDefinitions>
  <RowDefinition Height="Auto"/>
  <RowDefinition Height="Auto"/>
  </Grid.RowDefinitions>
  <ContentPresenter HorizontalAlignment="Stretch" Content="First Contact name:" ContentTemplate="{StaticResource FieldTemplate}"/>
  <ContentPresenter Grid.Row="1" HorizontalAlignment="Stretch" Content="Second Contact name:"   ContentTemplate="{StaticResource FieldTemplate}"/>
  </Grid>
  </DataTemplate>
  <DataTemplate x:Key="CustomerTemplate">
  <Grid Grid.IsSharedSizeScope="true">
  <Grid.RowDefinitions>
  <RowDefinition Height="Auto"/>
  <RowDefinition Height="Auto"/>
  <RowDefinition Height="Auto"/>
  <RowDefinition Height="Auto"/>
  <RowDefinition Height="Auto"/>
   <RowDefinition Height="Auto"/>
  </Grid.RowDefinitions>
   <ContentPresenter HorizontalAlignment="Stretch" Content="Name:" ContentTemplate="{StaticResource FieldTemplate}"/>
   <ContentPresenter Grid.Row="1" HorizontalAlignment="Stretch" Content="Contact name:" ContentTemplate="{StaticResource FieldTemplate}"/>
   <ContentPresenter Grid.Row="3" HorizontalAlignment="Stretch" Content="Address:" ContentTemplate="{StaticResource FieldTemplate}"/>
  <Grid Grid.Row="2" Grid.IsSharedSizeScope="false" >
  <Grid.ColumnDefinitions>
  <ColumnDefinition Width="*"/>
  <ColumnDefinition Width="*"/>
  </Grid.ColumnDefinitions>
  <ContentPresenter Content="Phone:" ContentTemplate="{StaticResource FieldTemplate}"/>
  <ContentPresenter Grid.Column="1" Content="Fax:" ContentTemplate="{StaticResource FieldTemplate}"/>
  </Grid>
  <Grid Grid.Row="4" >
  <Grid.ColumnDefinitions>
  <ColumnDefinition Width="*"/>
  <ColumnDefinition Width="*"/>
  </Grid.ColumnDefinitions>
  <ContentPresenter Content="City:" ContentTemplate="{StaticResource FieldTemplate}"/>
  <ContentPresenter Grid.Column="1" Content="State\province:" ContentTemplate="{StaticResource FieldTemplate}"/>
  </Grid>
  <Grid Grid.Row="5" >
  <Grid.ColumnDefinitions>
  <ColumnDefinition Width="*"/>
  <ColumnDefinition Width="*"/>
  </Grid.ColumnDefinitions>
  <ContentPresenter Content="Zip/Postal code:" ContentTemplate="{StaticResource FieldTemplate}"/>
  <ContentPresenter Grid.Column="1" Content="Country:" ContentTemplate="{StaticResource FieldTemplate}"/>
  </Grid>
  </Grid>
  </DataTemplate>
 <Style x:Key="{x:Type GroupBox}"
       TargetType="{x:Type GroupBox}">
  <Setter Property="BorderBrush"
          Value="Gray"/>
  <Setter Property="Foreground"
          Value="Black"/>
  <Setter Property="BorderThickness"
          Value="1"/>
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="{x:Type GroupBox}">
        <Grid>
          <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
          </Grid.RowDefinitions>
          <Border
                  BorderThickness="{TemplateBinding BorderThickness}"
                  BorderBrush="Gray"
                  Background="LightGray">
            <TextBlock Margin="14,4" Foreground="White" FontSize="12pt" Text="{TemplateBinding Header}" />
                 </Border>
          <!-- ContentPresenter for the header -->
      
          <!-- Primary content for GroupBox -->
           <Border CornerRadius="0"
                  Grid.Row="1"
                  Grid.RowSpan="3"
                  Grid.ColumnSpan="4"
                  BorderThickness="{TemplateBinding BorderThickness}"
                  BorderBrush="{TemplateBinding BorderBrush}">
          <ContentPresenter  Margin="{TemplateBinding Padding}"/>   
          </Border>
        </Grid>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>
  </Page.Resources>
  <Grid> 
   <Grid.ColumnDefinitions>
   <ColumnDefinition Width="*"/>
   <ColumnDefinition Width="*"/>
   </Grid.ColumnDefinitions>
   <GroupBox Margin="7" Padding="14,4" Header="Customer">
     <ContentPresenter ContentTemplate="{StaticResource CustomerTemplate}"/>
   </GroupBox>
   <GroupBox Grid.Column="1" Margin="7" Padding="14,4" Header="Dealer">
     <ContentPresenter ContentTemplate="{StaticResource DealerTemplate}"/>
   </GroupBox>
  </Grid>
</Page>

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
 <Window.Resources>
  <Style x:Key="Container" TargetType="{x:Type HeaderedItemsControl}">
   <Setter Property="Margin" Value="4"/>
   <Setter Property="Template">
    <Setter.Value>
     <ControlTemplate TargetType="{x:Type HeaderedItemsControl}">
      <Grid>
       <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="Auto"/>
       </Grid.RowDefinitions>
       <Rectangle Grid.RowSpan="2" Stroke="Gray" StrokeThickness="1"/>
       <TextBlock Grid.Row="0" Padding="4,3" Background="Gray" Foreground="White" Text="{TemplateBinding Header}"/>
       <ItemsPresenter Margin="4" Grid.Row="1" />
      </Grid>
     </ControlTemplate>
    </Setter.Value>
   </Setter>
  </Style>
  
  <Style TargetType="{x:Type HeaderedContentControl}">
   <Setter Property="Margin" Value="2"/>
   <Setter Property="Template">
    <Setter.Value>
     <ControlTemplate TargetType="{x:Type HeaderedContentControl}">
      <Grid>
       <Grid.ColumnDefinitions>
        <ColumnDefinition Width=".3*"/>
        <ColumnDefinition Width=".8*"/>
       </Grid.ColumnDefinitions>
       <Label Grid.Column="0" VerticalAlignment="Center" Target="{Binding ElementName=Ctrl}">
        <TextBlock Text="{TemplateBinding Header}" TextWrapping="Wrap"/>
       </Label>
       <TextBox Grid.Column="1" Name="Ctrl" VerticalAlignment="Center"
                Text="{Binding Content, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
      </Grid>
     </ControlTemplate>
    </Setter.Value>
   </Setter>
  </Style>
  
 </Window.Resources>
 <Grid>
  <Grid.ColumnDefinitions>
   <ColumnDefinition Width="*"/>
   <ColumnDefinition Width="*"/>
  </Grid.ColumnDefinitions>
  
  <HeaderedItemsControl Grid.Column="0" Style="{StaticResource Container}" Header="Customer">
   <HeaderedContentControl Header="Name:" Content=""/>
   <HeaderedContentControl Header="Contact Name:" Content=""/>
   <Grid>
    <Grid.ColumnDefinitions>
     <ColumnDefinition Width=".5*"/>
     <ColumnDefinition Width=".5*"/>
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
     <RowDefinition/>
     <RowDefinition/>
     <RowDefinition/>
     <RowDefinition/>
    </Grid.RowDefinitions>
    <HeaderedContentControl Grid.Row="0" Grid.Column="0" Header="Phone:" Content=""/>
    <HeaderedContentControl Grid.Row="0" Grid.Column="1" Header="Fax:" Content=""/>
    <HeaderedContentControl Grid.Row="1" Grid.ColumnSpan="2" Header="Address:" Content=""/>
    <HeaderedContentControl Grid.Row="2" Grid.Column="0" Header="City:" Content=""/>
    <HeaderedContentControl Grid.Row="2" Grid.Column="1" Header="State/Province:" Content=""/>
    <HeaderedContentControl Grid.Row="3" Grid.Column="0" Header="Zip/Postal code:" Content=""/>
    <HeaderedContentControl Grid.Row="3" Grid.Column="1" Header="Country:" Content=""/>
   </Grid>
  </HeaderedItemsControl>
  
  <HeaderedItemsControl Grid.Column="1" Style="{StaticResource Container}" Header="Dealer">
   <HeaderedContentControl Header="First Contact Name:" Content=""/>
   <HeaderedContentControl Header="Second Contact Name:" Content=""/>
  </HeaderedItemsControl>
  
 </Grid>
</Window>