253D user here.
Answers to your questions:
- There is a way to convert, but not straightforward - you’d have to do that via ssh, and do that every time the certificate renews. On top of that, would recommend you actually get rid of any connection on myqnapcloud.com - That domain is heavily and constantly being targeted by ransomwares for all kinds of exploits, so if you use the same certificate for both your QNAP and HA, both would be constantly under attack / scan. Anyways I guess you are aware of all these.
- No. crt format
- Yes you can convert crt to pem and then combine. See Using QNAP Let's Encrypt Certificate - #3 by lubosjerabek, if you absolutely have to. But again I’d recommend you try other domain.
- No. You’ll have to convert and combine and copy.
= = = Off-topic = = =
Your QNAP can do VM, and have enough RAM. I would recommend you backup your config and kill the container and go VM instead. See discussions around the end of this thread. People tend to agree the VM route.
One might think the container route gives you more control and taxes you less on resources. That’s true in general, but for HA, that also introduces a lot more maintenance tasks (in my view) than necessary.
So what do I do? I do HAOS in VM, which evidently barely occupy anything and would offer the flexibility to do let’s encrypt via add-on, or do ZeroTier also via add-on.
Well, now that I think about it. You can do ZeroTier or Tailscale on QNAP. Maybe something to consider. I am just sharing my experience, and recommends VM install route for an HAOS.