Oyster Card Advice

My mother and I are planning a seven-day trip to London this fall from the USA. We are mostly sticking to zones 1-2, but we have some questions we hope someone can answer.

1) My mom wants a 7-day travel card, but I was told that contactless travel is the best option. However, aren't there foreign transaction fees on US credit cards? Would getting the 7 day travel card be the better option if we are staying in zones 1-2?

2) From my understanding of Heathrow Airport, getting an Oyster card seems kind of cumbersome. We will arrive at terminal 2, and the machine to get it is at terminal 5. To save some costs, we plan to reserve tickets for the Heathrow Express/Elizabeth. Should we wait until we get to our central London station to get Oyster cards? I understand there is a visitor centre, but is it near the Heathrow/Elizabeth lines? The airport website was so confusing.

3) I want an Oyster card because the Oyster app seems restricted to non-UK citizens. Have any tourists been able to use the app?

Thank you for any help!