Hi and welcome to the community forum.
To register your phone you should be able to use either the My Fairphone app (sign in, then Home > Extended Warranty > Register your Fairphone) or on the web site: Register your Fairphone - Fairphone
If you have difficulties registering your phone, contact official support (see > contactsupport )
I personally hadn’t noticed any instability with the hotspot, but I rarely use it for an hour at a time. I haven’t seen this specific problem discussed in the forum, but have a look around. Again, you would do well to contact official support.
Regarding the flash-light auto-off: this question is dealt with in the following thread. The phenomenon is known and considered a feature to avoid unwanted battery drain and heating of the diode. They’re looking at the possibility of introducing a setting to allow the user to change this behaviour.