everyone accept the card companies missing out on their 2.5% transaction fees. One would thinks the smaller players like Wise and Revolut with they way they are set up could integrate easily enough
QR payments are great for both small vendors and those without cards because there are no transaction fees, which is probably why they're not yet more popular in Western nations. They're essentially faster payment Bacs payments. Obviously if you're looking at including foreign bank accounts it then involves the swift system and I don't know enough about that and the costs involved to see whether these instant payments are possible at no fee.
Think it will be just as much tondo with the foreign banks. PayPal offers QR payments but only on their own system. You probably should aim ant them and promptpay
it's normally at check in that the airlines check you have a return/onward bound flight as they can be held liable apparently, so if you have a return ticket you'll probably be fine but maybe not a bad idea for your own peace of mind. See which airlines allow changes free of charge in case your schedule changes