Regarding your points 1 and 2 you could have a look here if it answers your questions:
https://support.fairphone.com/hc/en-us/articles/14487777708946-Warranty-with-an-Alternative-OS
For your point 3 I’m not sure. So I won’t give an answer.
And regarding your point 4, if you are not experienced with unlocking, flashing and so on, I would use the return policy and take FPOS version.
No feedback to the /e/os issues, as I’m not familiar with it.
That’s what I did, and I’m quite confident with it at the moment.