Could Mai Trin return in Season 3?
Could? Yes. So could the Molten Alliance (we never killed their leaders) and the Toxic Alliance (they also remained active, but dwindled).
Honestly I expected the Toxic Alliance to play a role in HoT, but it was so lacking in story content that apparently even more important story hooks got cut.
But yeah, those three could return at any point potentially, but I don’t think they will because of the whiplash that Scarlet got.
Mai is currently in the Edge of the Mists, away from any clear means of communication, and there’s no real reason for the White Mantle and especially Lazarus to contact her.
I think if Mai Trin was related to Marjory, Marjory would have denoted such when they first met and Mai Trin was thought to be a good guy. She wasn’t wearing Aetherblade clothing, but she wasn’t under disguise either.
Stop treating GW2 as a single story. Each Season and expansion should be their own story.