Thanks for sharing it with us, Hatcat :)

But well, as it was pointed, the main problem with protectorate is that it's bugged for now.
The second problem, is that we simply don't know at all WHAT a protectorate is. There is some guesses, based on the word and some hints inside the game, but that's all. No definite explanation.

Additionnally, I think that the amount of pressure required to make a faction bend is hugely exagerrated. Even reduced to one province, with a puny army, after ten years of constant crushing defeat, they still ASK for something in exchange of peace/protectorate.

Well, if it's more costly to make peace, or to make them a protectorate, than to eradicate them, you can guess what's the option most commonly used...
As you don't have any penaly if you destroy a faction when you are ordered to make it a protectorate, it's what I do. No sense in giving back five regions and half my treasury while all the forces they have to counter my legions is 240 peasants...