Birthright:

Court 1 required for a Free Action Decree. A decree that has substance (eg, Dhoesone's) will cost 1GB. Wish-wash background stuff isn't a decree. I like BSB's thought on it, if it essentially has the force of law, it is a Decree (1GB cost) action.

That makes sense to me.

About half of turns are submitted more or less, to date

Mine is solid. Consider it submitted.

How does death plague work as a divine caster? I assume some different rules had to be adjudicated previously as if you tried to use direct equivalents you would need a level 5 temple and be death plagueing your own province only by default?
Also: 5 child limit per regent or per regent pairing? ie would an arrangement with another player yield 5 each or 5 total?

5 total in married couple.

If you marry a PC you are sharing the 5 kids.

Death Plague Divine does need to be addressed, I agree

I love how Marriage and Death Plague are put together. Great comparison.

Blight is a priest spell that harms the province it's cast in, too. Though it's less destructive. In either case, it's assumed the caster would be using it against the province regent, rather than be the province regent.

Though wizards have more reason to depopulate provinces (it helps the magic of the land recover from the damage done by civilization) the main advantage the wizard has is ley lines, which let them use the spell anywhere. Maybe priests of Arawn need an equivalent for their arcane spells, so they can use a temple 5 to cast arcane (offensive) magic in a province where they only have a temple 0. That'd make both their highest level temple and the (presumably hidden) temple 0 potential fracture points for any depopulation plan, similar to how it works for wizards.

(A lower requirement might make sense, too. Wizard spells need up to source 7, but no priest spells require temple 4s.)

Mass Destruction (Architect) and Defection (Rhyel) might require some slight adjustments too.