Sadly no, I don't think the chocobo gets anymore exp, but it just no longer has an impact on your ability to level. Though make sure you do the challenge log for companions if you plan to level your chocobo. That is the only two logs that grant the chocobo exp and not yourself. So make use of them every week.

Well if you're loving the Wyrm armor, the il90 myth gear will look the same just a different color. Also you can just do a glamour with it and you can keep it no matter the gear you need to wear. As for tomes, tomes come from anything that's 50+ content. And if you do Duty Finder Roulette (DFR), the low level ones grant tomes. I don't recall how much it grants. Best thing to think is that if it requires you to be lv50 you'll get tomes from it. The amount you get just varies, Roulette grant more, longer ones grant more and the harder it is the more it grants.