Post-Script: These new rubber feet certainly quieted the machine down, especially the front two, which also serve as bumpers for the space bar. While test typing last night on my granite-surfaced kitchen island, not a bit of slipping was noted, testimony to how well the new rubber grips. I would certainly recommend checking out the Typewriter Man's website, should your Corona need new feet.
As for the upper case shift problem, what I suspect had happened is that because, over the years, the rear feet had become permanently squished, making them wider than they should have been, they were interfering with the shifting mechanism. Then when I had the machine reconditioned some years ago, the local repair shop didn't have new replacement feet, and so they just adjusted the shifting to compensate. Subsequently, after the feet replacement, the shift mechanism was free to move further than before. The problem last night, as I indicated in the piece, was figuring out why the upper case adjustment screws weren't permitting enough range of motion, which ended up being debris caught in the bearing race for the carriage up/down motion.
I still haven't looked at the premature ribbon reversal issue, but I'll leave that for another day.