Power up Mewtwo

Yes, it's a Mewtwo. Leave it at 30 if you want to conserve resources for getting another one.

Hell to the yes. Almost as powerful as Groudon or Kyogre, unique to it's niche, and very hard to get, much harder than the rare candy or dust you'd power it up with. (In fairness, the hard to get refers to the EX pass, not the raid itself.)

What do you want to use it for? Psychic as a Machamp killer? Shadow Ball as a generalist? Focus Blast as a Blissey/Snorlax gym sweeper? All are good.

No. Instead, I'll use it as a candy collector until one with better IVs shows up.

Nope, wouldn't. Well, even if I would there is no ex-activity in my city, but assuming somehow I got access to a mewtwo below my standards I wouldn't. Yes, even worst mewtwo is pretty good, better than maxed hundo alakazam. However, somewhere deep inside it will always look incomplete. Besides, since having no access to Mewtwo I see no need for a one. As a psychic his charge move is pretty bad, too bad. As a generalist, at the end of the month there will be Rayquaza. As a ghost specialist, there is no need since alakazam departed, dark type fares well against psychics anyways. For gym offense, dynamic punch machamp is a thing and almost everyone has enough of those. Mewtwo is a fine pokemon that can do many things but not totally must.

To level 31 maybe

Im powering up one with 10/13/12. No regrets

Yes, I would.

Mine's also an 89, but 12A, 13D,15S, with Confusion/Psychic. I've powered him up to L32, no regrets, and will continue powering him up as and when my frugality with rare candy allows.