No the cp8l does not support tag
I would suggest an upgrade after 10 years
Pir wise i like the Honeywell or hkc
Wireless requires a hybrid panel
Again look at the hkc best wireless Imo or the honeywell
Both panels support apps too
It's difficult. In theory it should be ok and I have one myself without a false alarm yet.
The only way to be sure is to only arm perimeter protection and disarm the movement sensors
A volt meter does not prove the battery. You need to put it under load and take the reading again. If the battery is 11 years old id be amazed if it ok. Plus its possible that it has damaged the panel. Batterys must be changed before they fail
1. The 20 will suffice but the 50 will give you more flexability
2. You will need to add a wireless portal to the system
3. You will need to add an IP device then setup the app etc
If you are going to be using in that environment id recommend https://www.securitywarehouse.co.uk/catalog/honeywell-combination-rateofrisethermal-58-p-2449.html
And a 12v non latching base