My impression of Moon and Toffee and whatever their potential relationship was keeps changing every time we get more information. Still not quite sure where they're going with it.
I do think they definitely knew each other outside of the context of battle; Moon apparently almost crying at the end of the spell during the promo (and in the "Demon Buster" poster) confirms it for me. But I've always hoped it wasn't romantic and I'd like to think the teaser we just got implies it wasn't-- Moon seems really young there.
Definitely my first reaction, around Into the Wand, was that Toffee had been killing Butterfly women for a while and Moon put a stop to it by threatening his immortality and seemingly stopping any threats for the next few decades.
Then comments from the crew and stuff made me think Moon was at fault in the situation as well, somehow, and even that possibly Toffee wasn't as much of a psychopath as depicted in the tapestry. They were friends or diplomatic allies once, Moon maybe couldn't live up to higher ideals she may have promised him in the past, and Toffee's possibly justified reaction ultimately led to Mewman-Monster Accord stuff making things worse and him plotting revenge ever since and abandoning his original cause.
The teaser is definitely more hardcore about the "darkest spell" than I was expecting (somehow I didn't see it as a specific thing but just almost like dipping down fueled by hate), with some sort of dark contract with likely deeper effects, and seems to project that Moon may have been wrong in this decision and that it was not a good move for anyone involved.
I think your prediction is convincing in that it probably will be a combination of neither one being totally the victim, maybe both of them even killed someone, etc. Also I guess Toffee would've been a diplomat from Septarsis and that the history of that is related too, but though they've set a lot up it's hard to tell how it all fits together.