They have a super easy solution - do neither.
Just show her getting exposed to an otherwise undefined "mist", but leave it at that. Those who prefer her to be an Inhuman can just naturally assume it's Terrigen, and everyone else can just assume the mist was radioactive of some such, giving her a Spider-Man-esque origin story. If they decide later on that they want it to be one way or the other, it's super easy to just expand upon it and have some character properly describe what the mist was supposed to be. But otherwise, just don't bother explaining it - give the mist a bit of an unnatural glow, show her body reacting to it, and leave it at that.