If someone wants to backorder a domain name, can s/he do so if
the domain name is already in Redemption Period/Pending Delete-
Restorable?

I had an almost-heated argument with a friend of mine about this
and he says they can't. :D

eNom won't take backorders until the domain enters PENDINGDELETE, but other services (Go Daddy, Pool) will take backorders for any registered domain.

Yes you can, but you the days of backordering are over. It really comes down to how much you will bid in the last seconds of the auction at these "backorder" companies (If you really can call them "backorder" companies).