Gyarados leveling advice

1. Not for attacking, no. I might bump the 30 gyarados up if and only if I needed to for gym placement. Not the best defender, though, so that might depend on your loca gym situation.
2. You have excess candy either way, so may as well evolve. Again, whether you level depends on your local situation. Or for the heck of it. In terms of a great attacker or defender, I might go with something else.

You got lucky, I don't know I'd be too scared to get a Twister-Gyarados third time around.

I got unlucky the second time and now I'm preparing for a third evolution and fingers crossed it'll be good.

I prefer powering up both instead of going for a third gyarados with 33% chance of twister. You already have the best movesets anyway. At the end of the day, do you really need another one? 2 fully powered gyarados > 3 semi-powered IMO.

I suggest to save dust for other guys.
I don't like Gyarados in PoGo.
So I'm keeping my
(96% Bite Hydro) (98 B DragPul) (98 B Twis)
at about level 27~28, and saving stardust for something else.

Bite has no stab, and this alone makes it a bad defender. Furthermore it has 4x a weakness (lightning is not top, but everyone has Jolteons).

As an attaker it is pretty good, but I prefer Vaporeon as it has stab on Water Gun and I'm using my 100% Jewel Vap to attack.

I'd power both up to the max. Gyarados is useless as a defender, and you'll barely need more than two for attacking (I assume you also have some other mons)

I have three 32.5 (2 HP 1 DP), and use them both for gym stuffing and attacking when they are available. I would power these both up to the max and use them as such. Wish mine were 98%, they are all 91%.

1- Yes it's worth to power up Gyrados pass lvl30 because of high CP for gym placement.
2- I think you should max them both first before evolving a third one, you already have the best moveset.

Do you plant to use them as defenders or attackers?

Gyarados are bad defenders but, unfortunately, are popular defenders because of the high CP which gives good gym placement. It's 66 candies to go from Level 20 to Level 30. If you want another defender, take the Level 20 to 30 for sure.

It looks like you can get plenty of Magikarp candy where you live, so why not evolve another one. Heck, it's fun!

Also, I don't know why one would need Gyarados attackers. I find that there are much better options for pretty much any matchup.

I would max one of them, or maybe both if you like, but I would wait to evolve a 100% IV one.. what if you get another Bite/Hydro Pump and it just has shittier IVs, whats the point? I mean you have another Gyarados, if some peole hoard tens of dragonites/arcanines/gyarados due to their high CP, but I just prefer to have a couple of good ones.. you cant use them all anyway.
You have 2 close to perfect Gyaradoses, and the other charge moves arent really a good option, Dragon Pulse? Would you really pick Gyarados against a dragon? There are better options, like ... a Cloyster xD And Twister sucks so bad that when I evolved my 100% IV Magikarp and it got Twister I just gave it to the professor instantly!