What are the Mechanics Behind National Freespawns?

[Saulot]

I am not 100% certain, as it's been a while since I played R'lyeh, Pangaea, or Ermor... but IIRC I believe it's like this:
EA Pangaea (Maenads) is based on turmoil and dominion.
Carrion Woods is based on growth scale, forest provinces, and probably dominion as well.
R'lyeh is based on dominion.
Ermor is based on dominion, and possibly death scale.

For all of them, temples improve the type and/or increase the numbers. It's also possible that fortress may help (for Ermor or R'lyeh), but that's purely speculation.

As to your second question, there is no way to mod freespawning. However, there is a kind of approximation that you can do with the abilities summon and domsummon.
Each of them come in several flavors, and can be tied to specific commanders you could pick.

For example; I'll use an idea that I had for Urdheim decided not to use. If I wanted to make a Kobold Goddess of Fertility, I could create a pretender, and give her the ability '#domsummon1 "Kobold"', and she would spawn kobolds in that province whenever she was in her dominion, with the random number of them increasing as dominion went up.

Anyway, you should read up on these commands more carefully in the modding pdf.