Two external boxes would normally require two power supplies due to alarm current and standby times. Strobe consumes very little power relative to the rest of the box.

You should calculate the panels capacity with all components using standby current and then alarm current, making sure you get at least 12 hours standby and a full activation on your battery.

It's not as simple as the panel has a 1A PSU so you can load it to 1A.

Only sounding for a few seconds may mean it's in SCB mode and the battery is dead, but I am still thinking there is a tamper issue if it will not leave hold-off mode.

 

Okay, sounds like I’ll be needing access to both boxes to do a bit a rewiring.