.NET config files issues of proprietary soft

Question

I wrote an extension for big proprietary platform - DocsVision. This platform works using .NET 3.5 SP1, but I wrote my extension, using .NET 4 (I think, why not?)

Later, when DocsVision used Internet Explorer as host - I was added .config file to IE's directory with startup tag to support runtime of .NET 4 and all works good.

But now, in new version, there is new host - Navigator.exe with there own .config files:

Navigator.exe.config

<?xml version="1.0" encoding="utf-8" ?>
 <configuration>
  <configSections>
   <sectionGroup name="docsvision.platform.wpf.navigator" type="System.Configuration.ApplicationSettingsGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" >
    <section name="navigator" type="DocsVision.Platform.Wpf.Navigator.Configuration.NavigatorConfigurationSection, DocsVision.Platform.Wpf.Navigator, Version=5.0.0.0, Culture=neutral, PublicKeyToken=7148afe997f90519"  allowExeDefinition="MachineToLocalUser" />
   </sectionGroup>
  </configSections>
  <docsvision.platform.wpf.navigator>
   <navigator>
    <connection />
   </navigator>
  </docsvision.platform.wpf.navigator>
  <runtime>
   <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
    <!-- Probing path is used for fast loading of ngen assembly images -->
    <probing privatePath="Components;SystemCards;BaseObjects;Workflow;TakeOffice" />
   </assemblyBinding>
  </runtime>
 </configuration>

user.config

<?xml version="1.0" encoding="utf-8"?>
 <configuration>
  <docsvision.platform.wpf.navigator>
   <navigator>
    <connection DisableVersionMismatchNotification="false" LangName="" />
    <view ShowArchivedItems="false" ShowColorLabels="true" ShowDeletedItems="false"
            ShowNavigationPane="true" ShowPreviewPane="false" ShowStatusBar="true" />
   </navigator>
  </docsvision.platform.wpf.navigator>
 </configuration>

So, my .dll .NET 4 can't load, because of mixed mode. If I add to Navigator.exe.config the follow classic lines:

<startup useLegacyV2RuntimeActivationPolicy="true">
 <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
</startup>

my .dll will loaded correct and all my extension works, but it will have some problems in GUI and exceptions (on loading some other DLL) in other parts of platform.

What I do wrong and how can I use my extension without bugs in platform?

P.S. My DLLs firstly loads using COM, so I have this:

So, there is two domains into application. But when I try to do smthng action with my extension - there is an error of .NET 4 dlls loading. Why???