Bain

Playing as Bain, if I see a player has already resolved their Red card, can I play Blue and "guess" Red to get coins, even though the Red card is no longer in their hand?

Desmond is not a published character.

Bain's Alt Blue II reads: "You may have a hero in radius retrieve a discarded card. If they do, you may also retrieve a discarded card." Does "You may have a hero..." mean I decide whether they retrieve it, or is it their decision?

Yes, that's the reason for that particular wording, to be able to force retrieval.