My personal speculation on the Fuel Girls thing went like this:
We heard all of last season that the Fuel Girls contract was ending.
When Nightmare Fuel got announced a couple of weeks ago, there wasn’t any acknowledgment at all from the Fuel Girls social media accounts. I assumed, just like most everyone else on here, that meant they weren’t going to be involved.
Perhaps they weren’t “officially” back yet and were still negotiating parts of a new contract?
It’s seems possible to me that Universal was prepared to move on with different performers if they didn’t get a contract done in time. The show is really just a collection of differently skilled performers that are tied together with an original story.
This is all just my speculation.