If a mentor queues for the mentor roulette, there's no excuse for leaving. Don't like the dungeon? Don't queue for roulettes. Bad players? Help them out, that's why you are a mentor

There should be a severe penalty for mentors leaving mentor roulette since they are abandoning players that could need his help. I'd say getting locked from that roulette until next daily reset should be enough

What if you had to leave, like a real life emergency happened? In that case, I doubt you would be worried about not being able to do a roulette that gives little bonuses