I don't really pursue knowledge with the goal of mentoring other people, I do it to better play the game myself. If someone needs help with a topic that I have a good grasp of, I'll be more than happy to offer it. If they ask something that I don't know the answer to and no one else in NN replies (actually pretty rare on my server, mentors are usually falling over themselves to help sprouts) and I've got the spare time, I'll google it for them and then link them answer so they know how to look it up for themselves in the future.
Only if they ask for advice or it is bad enough that it is hindering the rest of the party, which is honestly very rare. Or if it's extremely obvious that they are confused about what they are supposed to use, like a tank wearing all cloth gear, etc.
Again, I don't watch guides or memorize anything for the sake of being a mentor. I do it to be a better player. I could watch a guide video for an encounter a million times and I would still be less qualified than a sprout who successfully cleared that encounter once.
Almost, I'm working on NIN which is 68 at the moment and will be my last job to 70. Again, this is something I do for myself and has little and less to do with being a mentor. Even after leveling them all, I would say I'm sufficiently qualified on only 2 or 3 DoM/DoWs to give anyone more than the most basic advice.
If I'm going to be honest, you seem just a little too caught up on the idea of being a mentor. I would try to dial it back a little because while it is a very worthy goal and the desire to help others does you a great deal of credit, it could make you come off as a little condescending to some sprouts if you aren't careful and try to act like some grand teacher sent to shepherd the poor little sprouts.
My advice boils down to this: focus on bettering yourself for the sake of being better. Let the knowledge and experience come naturally as you try to improve yourself. Treat sprouts like any other players (or people in general, for that matter,) with courtesy and thoughtfulness. Don't be a doormat or a heel. Offer help without being pushy and be patient while still knowing when to walk away from a lost cause (for your own sake.)