Indeed no strict negatives in law, alwas a minimum of 0,
Still if you can't use a negative value in add_turmoil it's not very useful, so the law bonus would be your best bet, still the easiest way to do this wouldn't even require the campaign script.
Give constantinople a hidden resource,
then just give constantinople a unique building a net law decrease somehow, either by a unique building with a law penalty or a line in any building you're sure it will have.
I would say the unique building as it allows you some for; of communication with the player.
Code:
law_bonus bonus -5 requires factions { byzantium, } [optional for none unique building] and hidden_resource Constantinople
law_bonus bonus -12 requires factions { turks, } [optional for none unique building] and hidden_resource Constantinople
law_bonus bonus -8 requires factions { egypt, } [optional for none unique building] and hidden_resource Constantinople
So that gives you some base turmoil, then just add the following line to the church buildings,
Code:
law_bonus bonus 2 requires factions { byzantium, } qnd hidden_resource Constantinople
etc, obviously you can play around with this a bit, but there is no real need for any campaign script involvement,
if you want Constantinople to go negative you could just have it start with say +5 law and have it drop again if you have a small church.
I hope the idea is clear.