Using HierarcicalDataTemplates in conjunction with TreeViewItem control templates

It seems that you are pretty close to what you're after. I tried to recreate your scenario based on the code you posted and I noted some problems with it (which of course are based on my interpretation of the code you posted)

• You are missing the ContentSource="Header" part of the ContentPresenter
• I think you are applying the ItemContainerStyle at the wrong HierarchicalDataTemplate level. It should be specified on the parent in order to affect the children (in your case SearchListViewModel).
• The default Template for TreeViewItem lays out the ContentPresenter in an Auto sized ColumnDefinition so the WrapPanel won't succesfully wrap unless you modify the ItemContainerStyle for the parent as well. I changed it to a UniformGrid in my sample below

With the changes from above and a few other things I got a result that looks like this which hopefully is pretty close to what you're after

I uploaded the sample solution here: https://www.dropbox.com/s/4v2t8imikkagueb/TreeViewAltered.zip?dl=0

And here is the Xaml code for it (too much code to post it all..)

<Window.Resources>
    <!-- DataSet-->
    <HierarchicalDataTemplate DataType="{x:Type data:DataSet}">
        <Border BorderThickness="3"
                BorderBrush="Gray"
                Background="Green">
            <TextBlock Text="{Binding Path=Tables[0].TableName}"
                       Margin="5"/>
        </Border>
    </HierarchicalDataTemplate>

    <!-- SearchViewModel -->
    <HierarchicalDataTemplate DataType="{x:Type viewModel:SearchViewModel}"
                              ItemsSource="{Binding DataSets}">
        <TextBlock Text="{Binding DisplayName}"
                   Foreground="Black"
                   FontFamily="Arial"
                   FontSize="16"/>
    </HierarchicalDataTemplate>

    <!-- SearchListViewModel -->
    <HierarchicalDataTemplate DataType="{x:Type viewModel:SearchListViewModel}"
                              ItemsSource="{Binding SearchList}">
        <HierarchicalDataTemplate.ItemContainerStyle>
            <Style TargetType="TreeViewItem">
                <Setter Property="HorizontalContentAlignment" Value="Stretch" />
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type TreeViewItem}">
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="Auto" MinWidth="19" />
                                    <ColumnDefinition Width="0.414*" />
                                    <ColumnDefinition Width="0.586*"/>
                                </Grid.ColumnDefinitions>
                                <Border x:Name="Bd"
                                        HorizontalAlignment="Stretch" 
                                        Grid.Column="1"
                                        Grid.ColumnSpan="1"
                                        Background="#7F058956">
                                    <ContentPresenter x:Name="PART_Header"
                                                      ContentSource="Header"
                                                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"/>
                                </Border>
                                <UniformGrid x:Name="ItemsHost"
                                             Grid.Column="2"
                                             Columns="3"
                                             IsItemsHost="True"/>
                            </Grid>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </HierarchicalDataTemplate.ItemContainerStyle>
        <TextBlock Text="{Binding DisplayName}"
                   FontSize="20"/>
    </HierarchicalDataTemplate>
</Window.Resources>
<Grid>
    <TreeView ItemsSource="{Binding SearchListViewModels}" />
</Grid>

Something I learnt a long time ago when trying to create a similar interface was that you are better using a ListBox than a TreeView.

Why?

1. If you only have one level of expansion (as it appears from your sample) you will a lot more control of the layout as you have a single DataTemplate to style.

2. It is lot easier to customize a ListBox than a TreeView as you do not have be concerned with the GridViewColumnHeader and GridViewColumnPresenters etc.

To get the expansion part (which is why you initially selected a TreeView), simply use a Grid with two rows defined and an Expander in the second row bound to the IsChecked property of a ToggleButton. See the example that I pulled from my Log Viewer.

<DataTemplate>
    <Grid Margin="0,0,0,3" Grid.IsSharedSizeScope="True">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="30" SharedSizeGroup="SSG_TimeIcon"/>
            <ColumnDefinition Width="120" SharedSizeGroup="SSG_Time"/>
            <ColumnDefinition Width="30" SharedSizeGroup="SSG_LevelIcon"/>
            <ColumnDefinition Width="70" SharedSizeGroup="SSG_Level"/>
            <ColumnDefinition Width="*" SharedSizeGroup="SSG_Message"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <!-- ProgramTime -->
        <Rectangle Grid.Column="0" Grid.Row="0" Margin="0,0,0,0" Width="16" Height="16" VerticalAlignme="Top"  HorizoalAlignme="Stretch" Fill="{StaticResource Icon_Timer}"/>
        <TextBlock Grid.Column="1" Grid.Row="0" Margin="5,0,0,0" VerticalAlignme="Top" HorizoalAlignme="Stretch" Text="{Binding Path=TimeStamp, Converter={StaticResource ObjectToStringConverter}}" ToolTip="{Binding Path=ProgramTime}"/>
        <!-- Level -->
        <Rectangle Grid.Column="2" Grid.Row="0" Margin="10,0,0,0" Width="16" Height="16" VerticalAlignme="Top" HorizoalAlignme="Stretch" Fill="{Binding Path=Level, Converter={StaticResource MappingConverterNinjaLogLevelEnumToBrushResource}}"/>
        <TextBlock Grid.Column="3" Grid.Row="0" Margin="5,0,0,0" Text="{Binding Path=LevelFriendlyName}" VerticalAlignme="Top" HorizoalAlignme="Stretch"/>
        <!-- Message -->
        <StackPanel Grid.Column="4" Grid.Row="0" Margin="10,0,0,0" Orieation="Horizoal" >
            <TextBlock Margin="0,0,0,0" Text="{Binding Path=LogMessage}" TextWrapping="Wrap" VerticalAlignme="Top"  HorizoalAlignme="Stretch"/>
            <ToggleButton x:Name="ExpandExceptiooggleButton" VerticalAlignme="Top" Margin="5,0,0,0" IsChecked="False" 
                          Coe="Show Details" Tag="Hide Details" Style="{StaticResource TextButtonStyle}"
                          Foreground="{StaticResource BlueBrush}" Background="{StaticResource RedBrush}"
                          Visibility="{Binding Path=HasException, Converter={StaticResource BoolToVisibilityConverter}}" />
        </StackPanel>
        <Expander IsExpanded="{Binding Path=IsChecked, ElemeName=ExpandExceptiooggleButton}" Style="{StaticResource CoeExpanderStyle}" 
                  Margin="10,0,0,0" Grid.Column="4" Grid.Row="1">
            <Border BorderBrush="{StaticResource DarkGreyBrush}" BorderThickness="1,0,0,0">                                
                <TextBlock Text="{Binding Path=Exception}" Margin="5,0,0,0"/>
            </Border>
        </Expander>
    </Grid>
</DataTemplate>

Can you see how much easier it is to define a header and expandable body. If you do have a need for nested data, add a Level property your view model (you are using MVVM aren't you?!) and then create a IValueConverter that returns a Margin (i.e. Thickness) to fake the indent.