Any ways to hook onto the pwr button presses without breaking the security model?
Some automation apps can hook onto the power button double-press (e.g. Tasker) without breaking anything system-wide.
Anything more than that (unlocking phone, changing profile) is something that is dangerous for any app to be able to do (imagine if you misconfigure it or it gets triggered by a malicious user, you wouldn't know it happened if the phone is in your pocket!), and probably requires root anyway, so this is the wrong place for it.
It takes too long for investigative work or arrest recording.
For investigative work, make a profile with what you need and stay in that profile, don't bother changing. Same for arrest recording, or use a spare phone or dashcam.