I've been playing the game for over 6 years, and the number of good mentors I've met exceeds the number of bad ones. Anyway, I'll address the different points you mentioned in your post.
- Telling other players they should have watched a video before joining the instance: Extremely valid in challenging content. I've never ever seen a mentor say this in normal content. I'm 100% with ZedxKayn. Don't go into challenging content blind in DF. That's a recipe for disaster. Mentors see this all the time, which is why they advise people to go to PF instead or to look up a guide. It's not a mentor's job to write walls of text to explain a fight when a video would be far better at doing the job. Unless your aim is to discover and learn the fight (in which case you should have the ability to actually figure out mechanics and do them correctly relatively fast), having the fight explained to you is no different from doing your homework beforehand by looking up a guide except that in the second case, you at least don't inconvenience another person by making them type walls upon walls of text (later fights get very complicated) and you also don't waste the instance timer. And this way, the mentors can help the party iron out the parts they're struggling with.
- To play an easier job if they can't understand how to play it: Again, valid advice. A mentor isn't expected to teach you your job (maybe they don't play the job and thus can't even help you with that). They might give you tips, though. In this case, starting with an easier job isn't a bad thing. It's not like they necessarily meant the person was too stupid to play the job, just that an easier start would be more beneficial and then when the person is more experienced, they can pick up a more difficult job. The other alternative would've been to read a guide about the job they're having trouble with.
- Not teaching mechanics or helping new players: Similar to point 1. In normal content, I'm yet to see a mentor not explain mechanics when people are struggling. I also think that mentors should only explain mechanics in normal content when people ask them to. Some people wanna discover the fights for themselves. Also, some mentors/players opt for not giving advice because of the way people usually respond to it (they get defensive).
- Many and I mean MANY of the mentors have left the instance after the second or so wipe: Honestly, I can understand this in normal content. If people can't even do the easy content, you'll most likely waste your time and/or get frustrated, especially if the mechanics have already been explained. People focus on mentors leaving, but even normal players leave instances when they deem it a waste of their time or they don't wanna put up with certain things. In challenging content, after a mentor explains stuff and notices people not following instructions after n tries, it's safe to assume it'll be a waste of time and they should be free to leave. However, mentors shouldn't leave the instant they see they got tagged in content they don't want.
The fact that the mentor system is the way it is is not just because of mentors. The community as a whole contributed to the bad state of the system. They kept meming about it and spreading the negativity when they most likely don't really know what they're talking about. To understand the struggles of mentors, you'll have to become one and walk a mile in their shoes. A lot of people have no idea of the horrors that take place in Mentor Roulette. Besides, some people actually treat mentors badly just because they're mentors and even when the mentors are just trying to help (true story).