This is my theory on how it happened. ^^^
Correct, good installers don’t do that. A common solution I’ve seen is to mount the panel 5-6ft above an outlet, run the wire down inside the wall and pop it out right at the outlet to plug in. Some even go so far as to install a recessed outlet so the wall wart is in the wall.
Many do reuse existing keypad wire as well. As long as it’s short/thick enough, that works great.
My preference is to not wall mount the control panel and use the back stand instead. There’s no security benefit in wall mounting it and it’s easier to maintain when it’s mobile.
It’s not as common (from what I’ve seen) for multiple keypad locations to share the same wires. It’s more common for each location to run back to the control panel directly.
We have a sufficient understanding of DC power. But we have to support these panels at scale. Most people don’t understand power and voltage drop as well as you apparently do. Qolsys’ own support takes a very conservative approach on this and our techs usually follow their lead to avoid suggesting people do things that damage their panel and/or void the warranty. If we (and Qolsys) were supporting professional engineers we would take a different approach.
That being said, electrons are electrons and power is power. Your IQ Panel can’t tell whether it’s coming from the manufacturer’s supplied transformer or your step down converter. If you know what you’re doing and are willing to assume the risk then you should be fine.