So how exactly does selling bounties work, with the bonuses? Do you leave a bounty undone in the non-bonus acts? Can you complete all the bounties regardless?

Just want to make sure I know how this works nowadays before selling any

Yes.

You do all except one in all acts.Inv ppl. Do all leftovers order to get bonus.profit.

..and do those leftover quests almost done that ur friends don't have to wait 1 hour when u searching plague tunnel 2.

This is not how it works.

You finish all bounties in every act 1-5, but don't hand them in to Tyrael. When you have all the people in the game, you just first hand in the bonus act and then continue to where the next bonus act comes.

Simple as that.