mfioretti on "Can different wp databases share the SAME UPLOADS FOLDER?"

"it is logical to believe both sites would first need to have identical settings related to thumbnails and cropping"

Yes! I had forgotten this detail, but yes, all sites would certainly have identical values for such settings, this is not a problem at all (not for me at least). Wrt this:

do not know how WordPress would act during the registration process instigated by 'Add From Server' if the upload being registered already had its corresponding thumbnails in place... there is no reason to believe a previous registration in a different database would be corrupted.

I have two comments:

1. The "Add from Server" plugin carries a big "not updated in 2 years, may no longer be supported" notice, so I'll try that as very last chance..
2. After thinking a bit about what you said, **I** expect/imagine that, if there is some problem in a scenario like mine (=each wp install has its own separate database) is not due to "database registration" itself (When php code of blog 1 writes anything to database 1, it has no idea at all of the existence of database 2, and no way to interact with it), nor to file permission issues (it's always the same user, the http server, that accesses the UPLOAD folder) but at one of these things:
   1. outside all the wordpress blogs, that is http-level caching behaving strangely, or inefficiently, if it has to cache two urls which are actually the same file
   2. wp code that creates thumbnails crashes/misbehave if they already exist
   3. wp code of the media library crashes/misbehave when you open the media library, and there are files on disk, never registered in its own database (because uploaded the first time, by another blog, that is registered in another database)

So, at the moment I expect that if there is any problem is in case 2.3 above, that is NOT seeing in media library of blog 1 stuff that is not registered/listed in its corresponding database. And going with "Add from server" doesn't seem a future-proof thing to try, not first at least. Must have a look at the media library code and db structures, like the get_media_item functions, I guess?

(of course, I will just go ahead, set up everything, see what happens and report, at some point. But I need a couple of free days for this and related stuff, which won't happen until I release other things. So now I can just prepare myself, asking and studying, 5 minutes at a time)