Hi Ross,
Unfortunately, this cannot be done. The Mobile API just allows to check the panel status, not the sensors. At the end you’re not interacting with the panel itself, just with the Securitas backend.
The Securitas backend just allows 5 kinds of arm/disarm operations:
- ARM: This means everything but the outside (perimeter sensors)
- PERI: This means just the perimeter.
- ARMNIGHT: Just those sensors that were configured as “Night Mode” when the technician was setting up the pannel.
- ARMDAY: Just those sensors that were configured as “Day Mode” when the technician was setting up the pannel.
- DISARM: This just disarms everything.
ARM, ARMDAY and ARMNIGHT are exclusive modes. You have either one or another.
PERI is optional and stand-alone, so you can arm this mode regardless of the state of ARM, ARMDAY or ARMNIGHT.
DISARM, as said, just disarms the whole thing, regardless of the states above. This means that you cannot actually for example go from PERI+ARMNIGHT to just PERI. You would need to disarm the whole thing and arm PERI again.
This can be done, but the code needs to be extended to fill that gap. I documented how to do it my API notes here:
https://github.com/Cebeerre/VerisureEUAPI
I’m certain that @segalion will be more than happy if you fork his github repository and contribute extending the code to make that happen ! 
Xavi