Q: Can the B2 DoubleBack card be used at all if another player has the Almanac in play?

or is the only option for a player who needs to change that linchpin to hope for an identity change to try and win?

A: Yes, it’s true that the B2 DoubleBack card is pretty much useless if someone else has the Almanac in play, but that doesn’t mean your only option is to try to change your identity. You could try to steal the Almanac with a Don’t Be So Gullible, or by getting the Dust Jacket, or you could try to trash it with Erased From Existence (or possibly You Are Fading Away if the Almanac is their only item in play). If nobody has the Almanac in play, anyone can change B2.