1) No. I don’t because it isn’t designed to be used with a keyboard cover
2). No. For the same reason as above.
3). Yes/No/Maybe so.
So as other users have commented, we don’t know the official cause of the issue, but it seems possible that it isn’t just crumbs that cause the failure, heat may also play a part in the degradation of the key materials and thus lead to key failure.
A keyboard cover will protect from crumbs, but likely will damage your screen because there is literally almost no space between the keys and the screen when closed; you’ll likely damage the anti reflective coating on the screen. Also, while the keyboard doesn’t have venting anymore, putting a rubber keyboard protector likely will kind of insulate and trap more heat in your machine, leading to higher temps and (if the rumors are true), lead to faster key failure from heat.
If you always leave your computer open, then this could help prevent crumbs from getting in, but otherwise I’d really suggest against it.