I think I've tentatively decided on version 4A below. Will try to order more ScaleTrax & Flexxbed this week. Unfortunatley, my grandmother is starting to decline & was moved last week, so dad & I may not be able to synch our schedules to get this done by Halloween. Will keep you posted, though.
Dewey: I've seen similar things at Lowe's/Home Depot, but they may not be wide enough for the corners. Thinking about maybe using some solid metal (~1/8: thick) L-brackets that I came across there to mount into studs & bolt shelf above. Could paint to match wall, shelf.
As always, all thoughts & suggestions are greatly appreciated.