It's a great theory but:
1) MJ isn't in hiding. She goes on television with Aunt Anna to promote Krakoan drugs.
2) Tombstone has Mysterio's (and Electro's) costumes. Now, sure, Quentin could have just misplaced a set of his fancy duds or they could be counterfeit costumes - but Mysterio doesn't seem the type to not keep track of his things and if the costumes weren't real, then they wouldn't really be worth anything to Tombstone in the upcoming gang war. This points to Mysterio being already incapacited, perhaps by whoever is behind the Super Adaptoid.