Re: nginx | nginx (proxy) and Apache | Apache
Answering on your questions:
a) Nope, no update is needed for 5.3.3.x versions, they run the same config.
b) I didn't delete any .htaccess, they are being simply ignored, but yes, I'm running nginx only setup and don't see any problems with apache configs left in the store folders.
c) Can tell anything about X-Cart hosting setups, but from my experience they don't run anything peculiar. I guess @AlexDyatchkov could share more.
Also, i can't provide any specific php 7.x settings. My own dev-config doesn't differ from 5.x one, but that is not a performance setup. I think that a performance-good configuration has to be tailor-made for the specific server and our recommendations would be useless.
I hope this helps.