Robert - That listing of C++ files was incomplete, my error. I do have the 2010 files. I've recently run repair jobs on all of them, using Microsoft downloads.
Here's a screenshot of what C++ files I have right now:

Yes - a complete re-install of 8.5 looks like it's called for, with all the backing up of custom data - argh - I would need help with that. I'm not up for the task yet - projects keep getting delayed because of the time I've spent on trying to fix things. But - if I still am really wanting to have 8.5 back, - I'll bite the bullet and do as you suggest.
John - This X3 installation is working fine, I'm getting work done. Naturally I'm clumsy with it since so many things were re-arranged. I'm working through the Groove 3 tutorials, I'm making my list of ?s as I work in it - Maybe I can just settle in and make this my only DAW software now. I can't decide that right now while I'm still being so unsure and clumsy with it - Need to learn it better first.
Example of ?s so far:
--Where's the pop-up for Copy and Paste, so you can select only automation, events in tracks, tempo etc? So far I'm only able to copy everything.
--How to lasso groups of notes in the PRV? I do that all the time. With the Smart Tool, I see how ALT turns it into Select, but it doesn't stay as Select to do the lassooing--? I have to go select the Select tool, grab the notes, then return to the Smart Tool to move them? I see F5 and F6 toggle between the two - but that's a bunch of extra fiddley steps to do this simple edit.
--The Snap To Grid is still Universal, like in the dreaded X1 release?? - And when I'm in PRV, with all the Grids on I can find, I still don't get a clean snap action - can't tell where the snap points are.
--I keep diving for the tool bar to get Erase, since I don't get how to do it with the Smart Tool.
--Yesterday I had an old plugin that was part of a project started in 8.5, and the plugin (FX3) worked - today, it doesn't work, Cakewalk reporting it as broken. -- ?
And so on.
Yeah - maybe once I've gotten X3 under my belt, I'll just only work with it. - I'm just not thrilled yet - growing pains.
Randy