Understandable. I expect you already have a 32bit installation of some version of SONAR, use it when necessary otherwise start swimming in the 64bit end of the pool. C3 may be updated at some time down the road making 32bit installation easier on a 64bit OS but I doubt the issues you have with shared locations will ever change. They have existed since the initial 64bit release of SONAR. There is no reason to expect losing the ability to work with old projects but there will be issues such as missing 32bit DirectX plug-ins. Bounce/Freeze in the 32bit host is your friend. Everyone with projects created in 32bit DAWs has worked through the same issues.

If the problem is the shared user directory would creating different Windows user IDs for 32bit and 64bit SONAR be any help?

32-bit plugins will have a button in the taskbar that you can click to re-open them.

I'll look for that next time. I am usually using Full Scrn, so might have missed it. I did try windowing each thing to finally get to a desktop which didn't have it either. I'll check the task bar.
 
I also just lost the display in the window for my IL-Sytrus_64b VST. The window was there but no graphics for the VST. It had been working just fine and I have no idea what happened, but I couldn't get it back. I had to delete it in the browser, recreate it, and ended up losing the midi clip driving it. That shouldn't be a bridging problem, though I'm just starting to turn on the 64b VSTs in FLS_64 which may have issues, though that transition has been pretty straightforward comparatively.