I will go straight to your bullets:
#1 - I just tried doing the same and managed to do so successfully. Here's what I did:
1. Created a new web app targeting .NET 4.5
2. Opened the default page in design mode
3. Opened the toolbox, clicked "Choose Items...", browsed to the Bin45 folder (the version of our controls should match the version of our projects - .NET 4.0 - bin40, .NET 4.5 - bin45)
and selected Telerik.Web.UI.dll (that is the only assembly actually containing controls)
After that, both the toolbox and design surface worked perfectly. I was able to drag controls onto the design surface, tweak them through the smart-tags and they ran perfectly at run-time.
#2 - As you might imagine, our controls have no dependency on Visual Studio, so there is no reason for them to require a particular version of Visual Studio. Loading your project in VS 2012 should work (if it is a simple web app - database, architecture and various other project types are not supported), but will upgrade it in the process and make it useable only through VS 2012.
#3 - You will not be able to use JustCode in VS Express, as it supports very limited third party extensibility.
Let me know if that helps.
All the best,
the Telerik team
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