Turns out, the "Demand Gavelkind" decision wasn't touched at all, only the potential for getting Gavelkind itself. Made a bit of a tweak which I'll attach here. (This mod includes the previous I posted, because both deal with Succession tweaks.)
Note that once again, you'll be able to set your gavelkind preference at game start. There's once again 3 options: "Kings and above" (The default that doesn't change anything), "Dukes and Above" which allows Dukes to take Gavelkind, and "Counts and Above" which allows Counts to take Gavelkind. This way you can move either back to vanilla (Well not barons, but maybe I'll add that it later, not sure if they have gavelkind in vanilla, never checked), or ensure that small, growing realms don't shatter until they at least attain Duke tier.
In case you want to use this with a previous save, you could try the following:
Step 1) Save your game without compression (Untick the box next to naming the save, this can't be done on Ironman)
Step 2) Find your save file and open it with a text editor. Notepad++ always recommended
Step 3) Near the top of the save file, probably starting at line 23, you'll see the Game Rules listed out. Add one (just one!) of the following:
For Dukes and above:
ek_gavelkind="gavel_duke"
For Counts and above:
ek_gavelkind="gavel_count"
Step 4) Save the changes and play the game, shattering your vassals, seeing them driven before you, and hearing the lamentations of their women.
If you want, you can also add in ek_gavelkind="gavel_default" which is the Kings and above option, but I'm pretty sure the engine assumes that by default if nothing is specified so it shouldn't do much if you do.
You could also add in the game rules for the Agnatic-Cognatic portion, but it wouldn't do anything, as that mod only changes the chances of countries picking those succession types upon game start, so it shouldn't affect anything in an ongoing game.
EDIT: And typically, I forgot to add the file. Here we go.