I do not have data on my phone plan. Will Apple Pay work without data?

Payment authentication is done over the internet, so a wifi connection is good enough for that purpose. It doesn't need to be done over a cellular connection, nor is it a requirement to have a cellular data plan (I have Apple Pay working just fine on an iPhone 6 plus without cellular service or even a SIM card in it). And the internet connection is only a requirement for setting up a new card on Apple Pay and for online purchases. For in-store purchases using Apple Pay, no connectivity is necessary at all because the payment info is stored on the phone itself (or the Apple Watch) and the authentication from the bank or card issuer is obtained by the merchant's POS terminal just like it is when using a regular card.

Yes. Apple Pay doesn't require a cellular data plan or even cellular connectivity (I have it set up even on an iPhone 6 Plus without any active cellular service or even a SIM card in it), all it needs is an internet connection. And only when you're setting up a new card or making an online purchase. So in either one of those cases, if you don't have a data plan you may still use wifi for an internet connection. Other than that, when you pay in stores no connectivity is necessary because the payment info is stored on the device itself (i.e., once you've set up Apple Pay you can use it in stores even with the phone on airplane mode).

Yeah that wouldn't be surprising. Apple Pay is not available everywhere, and even in the countries where it is already available it's not supported by all of the banks. It all depends on which country you live in and, if it is a supported country, on which bank issued your card. Some banks even restrict which type of cards they allow to be added to Apple Pay and which not. For instance, many banks do not allow corporate cards to be added to Apple Pay, and some other banks only allow credit cards to be added to Apple Pay. Just double check the list of countries on apple.com and, if necessary, check with your bank to make sure you didn't get the 'card not supported' notification by mistake.

I don't believe so. Apple Pay asks for authentication from the bank/card-issuer each time it is used. Same as the payment received authorizes the card, requiring an internet connection.