(1) How many Ruby versions... Just the 'A' version ??
That's all I see on my system.
(2) How does the Mac SketchUp edition decide what Ruby version to load ...
Is it hard-coded, OR is there a plist file ??
There are several plist files that might be relevant, including:
/Applications/Google/SketchUp 7/SketchUP.app/Contents/
Frameworks/Ruby.framework/
Resources/Info.plist
Versions/A/Resources/Info.plist
However, a quick inspection didn't reveal any pointers in the plists I examined. I suspect that the determination is made by following this symlink:
.../Ruby.framework/Versions/Current -> A
(3) Could you 'juryrig' SketchUp to load another version ...
(4) If it works... can it work for the latest v 1.9.x ??
Maybe, but I'm hoping that someone with access to and/or knowledge about the internals of SketchUp will jump in and tell us what's going on. I dislike snipe hunts...
FWIW, I think we're more likely to be able to use a full copy of MRI 1.8.5 than any later version (less differences to get in the way).
(5) The RVM issue. I can't use RVM, ...
I suspect that RVM could be made to work under CygWin, if that isn't too weird for you. If you're interested in pursuing that goal, I suggest that you chat with the folks on the #rvm IRC channel...
I don't know whether RVM has an override to set a custom path, but I can't imagine it would be very hard to add on, if not.