Which Dratini to evolve?

The second Dragonite will serve you much better in the long run.

Those Vaporeons are decent candidates for the Ex-Raid.

Mewtwo is relatively easy to catch (with GRBs) even if you lose out on damage balls. Congrats in advance from a trainer who will probably never get one.

Option 3.

You are going to be relying on the other trainers to take down Mew 2. Plan for the catch phase of the encounter instead.

In a few months, you'll be the one doing good damage at another ex raid. And after that, maybe you'll be carrying the raid.

So it goes.

Depending on how many CTM's you have, it might also be worth it to try and get Crunch on your Gyarados for a double dark moveset, which will be useful against Mewtwo.

Your area might be different but have done 6 ex raids & never got past my second mon so don't worry too much about it unless there's a poor turn out. Mewtwo is easier to catch than the other Legendaries & mine went in in the first few balls so throw solidly & you should be ok. Larvitar CD coming up so you should be able to stack plenty of candies to begin your big t army in preparation for your next ex invite, good luck 👍

I'd go with option 3. Save the dust and everything for a really good one.

For the damage balls:
<5% damage = +0, 5% = +1, 15% =+2, 20% =+3

In a group of 20 (5% average). U are not likely to make the cut with or without Dragonite.
With a group seize around 10 (10% average) u should get the ball. with or without Dragonite.
Same goes for Latias.

Just tell your situation with other trainers at EX raid before it happen ( come early half an hour before help)
Depend on playing style of trainers in the area, but in my experience most trainer willing to help for opening dex other trainer cos most already have atleast one ( that is the case in my city)
Some still blindly go with a lot of trainers, if you lucky you may find trainers split by a team colour that will help you gain more ball by team contribution
Some even split by team and divided again by 5 trainers to optimize personal ball. They ussualy strong enough to 3-4 men mewto by themselves and dont mind to carry trainer for opendex (again tell your situation)
In the meantime, better bring your highest CP Mon first...
For the future you can try to power up you meta pokemon to level 30, or if you wanna make it simple power up until it need 5000 stardust (almost 30) that gonna be your base
After that you can gradually power up that you use often when you have a better knowledge. There is breakpoint, dps, tdo and such but keep it simple at first

Don't care about the Mewtwo counters because it will anyways be defeated if you are more than 6 people. About the Dragonite, evolve the hatched one if you are about to spend Stardust and Candy on powering up, or else evolve the lvl. 25 one.