Someone spammed my original thread with the same response several times so it’s no longer possible to reply to it.
It was established that issues #1 and #3 are related. For some reason, my HSTS and Cache-control headers are being overridden.
The conclusion regarding #2 is that you don’t need to configure anything SSL-related in Cloudflare.
Issue #4 was answered: Workers, which is a paid service.
Issues #5 and #6 were not addressed at all.