in a rubber monster suit
I guess it's possible, but it's not how I read the situation. Martellus is very sick and very vulnerable right now, and Seffie is clearly exhausted and totally vulnerable herself. If this is some Machiavellian plan it's a pretty bad one. The whole thing to me looks exactly like what it says on the tin. They're desperate for Agatha's help, and that's it. From a Doylist perspective, I think Phil is done with certain plot lines and he's cleaning them up now. (This implies we might be getting a new plotline, like some clues about goddesses and Lucrezia, courtesy of Albia.)
For that specific scene, my take on it is that Seffie is telling Martellus what he wants to hear. She actually disagrees with what he's doing, and she'd rather stop him, but she can't, so she settles for playing a longer game and let's Martellus do what he's going to do anyway. The fact is Seffie seems to have a lot of empathy and cares for Agatha, but also cares for her brother. So helping both is not beyond imagination. Actually it's totally in character for a non-sparky, normally socialized human being.