The trigger warning is gotten when the faction you're attacking is less than 50% of your size, iirc.

It's also lasts 12 years after the warning. The 2 first years are the allowed to cease hostilities time, then the next 10 years gives the excom.

Don't remember about how the death of the pope or king affects the warning, but it's reasonable that it behaves the same way as the excommuication itself.

Bypassing the warnings are easily done by going full defense and then attacking another nation (they always seem to gang up on you) in the meantime.