Hello.
Firstly, you didn't say which model and manufacturer.
Some more points:-
*I would expect if you interrogate the panel it will tell you which battery.
*Just because the PIR's are lighting up, that doesn't mean you have diagnosed where the low battery is.
*It should trigger a tamper on removing any lid, if you get an alarm sounding will depend on the controls.
*Batteries are all different, but probably a special Lithium one, not your standard Alkaline type.
*Yes you should replace all the batteries at the same time, again these are different depending on panel.
*In theory yes the PIR's can be removed and refitted, again alarm will depend on controls.
I think your best option is to have the alarm system serviced properly by a local approved company, if you have the details of the installer try them first.
This way you can make sure everything is working correctly and they will have all the batteries available to do the job in an hour or so.
They can also give you some advise on your PIR's and the decorating.
The batteries will need replacing on a regular basis so find out about service intervals and maybe a service contract.