Yes it's possible to change your own batteries but possible and a good idea are 2 different things. I wouldnt get involved in that especially if your insisting on annual changes
I would leave it to the maintainer but batteries should last more than 2years. Manufacture spec may say much higher but I'd still go every other year. If its failing before that I'd replace the device. Also as your 7 years into device design its close
If a lower graded i&has part is fitted to an i&has then it lowers the overall grade however fitting an unrelated switch to signal something else isn't i&has
Smoke detection in yhis case would be classed as ancillary devices and would not need to be disconnected.
However they are not the same as a 5839 fire alarm. But i have smokes and Co on my own alarm
Bear in mind that if you extend that time your bb is still going down its just waiting till its down for the time set. Most people don't use the Internet 24/7 but your alarm is.