Let's address your points.
1: Sprouts should be able to queue for whatever they want to queue for regardless if it's the norm or not.
2: Mentors must previously clear all content before the roulette is opened to them. You have cleared the fight and thus have a general understanding of whats going on. No one is expecting you to know everything.
3: Whilst it's true more people queue as DPS on mentor there are still plenty of us Tank healer Mentors out there to make up for it. Regardless, I'd rather have an experienced player on DPS than on Tank as most Tanking in this game is stand still and hit boss. I've never in all my time doing mentor roulettes had as issue with who was or was not a Tank / Healer. Some are better than others, but as long as they are willing to read chat then it's not issue.
4: Using Food or Potions is not required in any content outside of Ultimate and first few week Savage. Extreme is no where near that level, also due to the numbers crunch and ilvl scaling you massively outdps all Extremes up until at least Shinryu. For ARR and HW you absolutely nuke the trials to the point you can tell new players to focus more on mechanics than damage because it doesn't matter.
5: Whether people leave or not literally doesn't matter, once you are in you are in, the game will try and fill in progress parties faster so just getting in is a massive boon. As for the teaching part, just make some guides online if you want to be a teacher, when you apply to be a mentor it tells you there and then that you are there to back fill and do so in a positive manner. No where does it say you have to be always teaching players all the boss mechanics. That's just what you implied when you put on the crown.
If anything they should just remove all rewards to do with Mentor so we don't have to listen to people complain because they got something they didn't want as it takes too long all while they blame it on not being what they signed up for.