As usual, quest givers and other NPCs will talk to you during side-quests, except it doesn’t make sense when those characters were killed (e.g. Calypsos) or, to a lesser extent, captured during the story (e.g. Tannis).
I think the whole thing would look better if there were some points of no return after which some side-quests would be disabled. It’s not like it would be much of a problem for completionists because you have more than 1 playthrough and ultimately you can reset your progress in TVHM anyway.