Bain

With Bain's Drinking Buddies or Another One! ("You may have a hero in radius retrieve a discarded card. If they do, you may also retrieve a discarded card"), if the other hero doesn't or can't retrieve a card, can Bain still retrieve one?

No. Bain only retrieves if the other hero did.

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.