Yes, compels will work. But compels will also work for preventing a loop garou from changing when its not the full moon and being forced to change when it is, and yet they still get a rebate from human form--rare/involuntary change. How is what I'm doing different?
As I said, rebate Powers modify normal Powers. Your Oathbreaker Power does other stuff, like hitting you with mental stress.
That said, there's a bias on this board towards using Compels over rebate Powers. There's a good reason for that bias; Compels play better. Even in the case of the loup-garou, it's healthy to use a fair number of Compels.
Compels are at their best, and rebate Powers are at their worst, when it's unpredictable how often your weakness is going to matter. So "you're weaker when you're facing the person you broke your oath to" is a classic Compel thing.
Edit to avoid triple-posting: How many shifts of power would be needed for a Darkhallow? And do you think one could be cast at Chichen Itza, now that all the vampires are dead?
I don't think Chichen Itza could sustain one. My understanding is that you need to eat a whole bunch of souls to make the thing work, so there needs to be a large + dense population present.
As for the shifts, it's really up to you. The game has never made anything about the Darkhallow clear, as far as I can remember.