Power banks are not allowed to be used on most long haul flights I've been on (trans-pacific), and when transiting through China they even confiscate them if they are over a certain capacity, but all flights have AC plugs at each seat.
Yeah, make sure it's not above a certain voltage. You can find this information on the website of your airline. I think it was 12500mAh for most, but I could be wrong.
Not sure about US companies, but I've never seen an AC plug on European and Asian companies' airplanes. They do have USB ports but they're underpowered (barely enough to keep my battery at the same level while playing music on BT headphones) or straight up don't work. Ask a flight attendant in the latter case, they'll reboot your screen which might fix the issue.
Definitely the neck pillow! A nice proper one if you have the space, but at least an inflatable one!
And a big bottle of water once you're through security.