Afaik it's advised to just add PROP_HANDLERS in modules when needed - but can I predict in which order they are executed, for example the core PROP_HANDLERS before the modules?
In my current case I'd like to add a PROP_HANDLER(PROP_ROLLING_PITCHING_LEVEL) to auto-switch the af points from portrait to landscape and vice versa by reading the "struct rolling_pitching level_data" that is initialized in the core PROP_HANDLER.
Think of a prop handler as inert until its property changes, then it fires (runs).
Sounds like you just need a simple if statement in that prop handler, so when pitching level changes, check if it's over some threshold to detect moved to vertical/horizontal, and if it's over that threshold, call your function to switch AF points and update some status variable so you know the current state (vertical or horizontal).
This sounds like a feature already implemented by Canon in the 6D (and 5d3 probably too).