So, what you essiantially want is having competend people who have a good knowledge of the fights to queue for them again, so newbies can clear it in DF...?
I mean, I do see where you're coming from but I also see a lot of issues with that. You pointed out yourself that clearing several fights is very unlikely when there are lots of new people who dont know the fight - a roulette wont fix that either. We also have fairly simple trials and raids and more difficult ones. Clearing T1 or Shiva EX in DF isnt (or at least wasnt) to difficult, even with new people - but compare that to fights like T13 (that kinda requires a certain set-up, or at least used to, when done synched) or Ramuh (a primal that requires great amount of coordination).
I could actually see a LOT of people bailing on the more difficult trials, because a 30 minute lockout will still be shorter than trying T13 with two scholars and three dragoons.
I also dont understand your point about "people knowing what they're queuing for"?
To unlock your mentor roulette you must have cleared all those fights - that would be the same for the EX-trials roulette, I assume? Neither ensures that that person actually knows all the fights inside out - after all they could have cleared them unsyched aswell, so I dont see your point?
Or did you mean that this roulette would make sure that people who are queuing for a fight did their research before? Nah... nah...
I understand that you'd like to do those fights the proper way, but I doubt a roulette will help here. Doing this the proper way means: Learning the fights all over again. Getting your clear with 7 people who know any fight already isnt "clearing it properly" either, its a carry - I dont wanna sound rude but when you say "We cant clear this fight with 4 new people" it sounds a bit as if you were hoping the DF would put you in a party with 7 experienced guys which would make any fight a piece of cake.
Doing those fights "properly" in DF will never be a real thing, I'm afraid - thats what statics are for...