1. What level should I be on right now?

It's been awhile since I last played that part, but I don't think I was over level 20-22 at the most.

2. Who should have which job right now in order to beat him the easiest?

Well, the game leads you to use a Scholar, but a Geomancer is also perfectly effective (you can even get a lucky Shadowflare).

3. which jobs should I be using at the very end of the game? I plan on having a knight, sage, dragoon, and black belt. I think I'm going too offensive.

That party would work, although changing your Sage to a Devout would be recommended to maximize your recovery power.

4. Is getting onion knight worth it and is it really recommended?

It's worth it only if you're a completionist. If you want to really level them up to level 99, then they become the strongest class in the game, but this is only if you have to do everything.

5. What level should I be on at the end when I fight Cloud of Darkness?

With a good healer, anything in the 50s range is fine.

6. I really don't want a summoner. Is it really necessity?

Not at all. Although the effects of summons are helpful, you are much better off with a White Mage or Bard for your recovery and status effect needs. Offensive spellcasters are also not a necessity, and grow stale after awhile.

7. How do you think Ingus should be? A mage or fighter. I see him more as a fighter.

Logically, since he's a Sasune soldier, he would be a melee fighter of some sort, but any character can be any class proficiently.