As above it was many years ago I'd assume the same thing will happen with ring blink etc. Also iirc it was a Motorola rebadge from a visonic unit with firmware that could only use their service sold at a discount on product hardware cost to recoup on the service. Plus to be a customer you must be paying bt to be one?
As above it should be one sensor per input. The old old regs allowed 10 per circuit if latching but that was a hangover from the 70s. I'm a big believer of if you can get a wire to it you should but 10 devices on a wire is worse. If you can't use some form of Id then I'd suggest wireless is better in this case. You could use addressable with hkc and keep the single sensor per point etc
As above its probably a test call under tel no 1. Its a requirement for grade 2 communicators to test the alarm path at least once every 24 hours to prove it could use it if needed
If you dont know the code and it hasnt been serviced in years your gonna struggle to stop it without fully decommissioning it.
Alarms need a little looking after or they cause issues as you have found out