Okay... I recommend that you seperate the ability bonus from the unit type (for ease of reading and changing things in the future).
like this (This is not necessary for the game though)
You are correct that there is something already called binding and you should rename your <AbilityBonusOption InternalName="BindingDemonSpawn">
or something. Then you also need to add to ceresa sovereign the line
this will allow Ceresa to get the actual ability.
Also, i recommend that you add to the ability bonus option what I suggested before about the gamemodifier with just a provides tag. It will give you the tool tip when you hover over the icon in the unit screen.
I didn't check your spell. I only did these few changes to see the ablity in game.