That seems like a reasonable theory. I was kind of doubtful about it at first, but you make some very good points. However, we have seen Muse and Ed interact, though I'm not sure how long ago that was so he could have been Hijacked between then and now.
But I personally think that Ed isn't Hijacked. However, after reading what you said, I think it's highly likely that Hijack will at some near point in the future (for the characters, at least,) try to Hijack Ed. HOWEVER however, we have not seen Hijack hijack anybody except for a normal person (and probably/maybe a teacher, who, while eccentric, are not spectrals) so we don't know yet whether Hijack can hijack someone who already has a spirit. We also don't know if he can Hijack spectrals at all, or whether he can hijack mediums or not, so this leads to a lot of possibilities for what's going on and what can go on.