Apparently as long as you use Signal at least once every 7 days from a linked device, you should be good: https://support.signal.org/hc/en-us/articles/360007059792-Si...

Still, given that your number is used as a primary identifier, I'd avoid using it in that way for an extended amount of time. Among other things, I'm not sure if it's possible to re-register using just your phone number and PIN (but not access to SMS-OTPs on the associated phone number) in case you lose your own device, for example.

No, this is exactly the kind of thing registration lock is intended to address.

If you enabled Registration Lock, your account cannot be hijacked by sms, provided you’ve been actively using your signal account within the last week.

There’s an automated keep-alive for the case that you still have signal installed but haven’t been sending/receiving any messages.