Jusr as Ward Against Faeries of the Waters ReAq, and Circular Ward Against Faeries ReVi are both valid spells, I don't think that a mentem ghost specific version invalidates this spell.
I chose base 4 because it is "change a fire so it is completely unnatural" When someone reads the spell they'll go to the MuIg guidelines first I think that this will be clearer. As you said, the level doesn't change.
Now that I've got a few ok's I'm going to copy it back to the other thread here:
edit: I wanted to add how that I appreciae how great you all are. Every post here is foused on making a better spell. thanks