Ok so as funny as calling Alastor a fraud is, it's also missing a LOT of nuance in his deal with Rosie and this nuance is. Kind of crucial. So I think it's important to clear up now that:
Alastor did not "cheat."
There's a very small detail about his deal with Rosie that actually makes it VERY different from the other kinds of soul deals in the show. Most of the other soul deals are very straight forward, classic selling-your-soul type things: you are given (blank) in return for selling ownership your soul to the person giving you (blank). Or, in slight variation: your soul is taken by (blank) in exchange for whatever you asked of (blank). It's kind of one-and-done thing. You make the deal, you're contracted until one of you dies permanently (presumably) or the deal is forcibly broken with. Simple, right?
This is NOT what Alastor's deal is. His deal isn't giving Rosie his soul in exchange for power, it's Rosie giving him power and keeping his soul until his debt is paid. There's a very clear "out" clause. He didn't sell his soul, not really, it's just being held from him (like a frozen credit card, if you will) until he pays Rosie back by completing whatever "task" she has for him. Granted, we don't know what this task is or if either of them even though, so this could be a loophole Rosie exploits to essentially keep him on a leash forever, but I really doubt that she doesn't have some modus operandi that Alastor is integral to and will honor their agreement afterwards.
This very particular detail is what makes their relationship so interesting, but that's for another post. My point in bringing this up is that this isn't Alastor boosting himself on someone's power, not really. After all, it's pretty fair and completely transactional. He gets something from Rosie and he then pays her back for it, simple as that. To put it in a more mundane scenario: if you borrow an egg from your neighbor, you're probably going to let them take a measuring spoon next time they ask, because that balances it out. They just haven't done their balancing act yet. You're not dependent on your neighbor just because you borrowed something once, the scale just hasn't been settled, so to speak.
Because, really, Alastor doesn't NEED Rosie's power. Even with his staff broken (which seems to contain the magic Rosie gave to him), he's pretty damn powerful. Rosie definitely played her cards right in that regard, because Alastor very likely would've become an Overlord without any boost from her just fine, it just might've taken a few more years. He's no fraud. He's not even "cheated"—he's leveraged himself a head start that he's going to repay.
And it may seem incredibly skewed in Rosie's favor, but it's not really when you look beyond the soul-owning thing. Not only is it temporary, but they're both, at the end of the day, getting the same thing: power. Souls give you power, so Rosie gets power. Rosie gave Alastor power too. It's mutual. And with how she talks about him being "the one" (for her plans) compared to Alastor's ability to stand on his own, I would even argue that she might need him far more than he needs her. But that's also another post.
TLDR; If anything, this wasn't even really a boost, it was a business deal.
