Hi there. I've done over 2000 mentor roulettes.

1. I used to queue mentor roulette more than once per day. Now I queue it only if I need tomes or I am bored.
3. It varied when I was doing it but it was mostly like this:

Healer : Story dungeons and trials with the occasional guildhest mixed in. Rarely got EX trials. Got a lot of level 50 MSQ dungeons

DPS: Guildhests and EX trials.

I can't say for tank because I never ran any on tank.

I think the requirements are fair but I also agree with the above poster that Extreme roulettes should be taken out. It gets frustrating and old when you try to teach people old extreme fights and you get met with snarky remarks, people who don't listen, and people who are extremely (heh) rude to you for trying to help.