Your puzzling thoughts were the exact same ones that have kept me awake
I have been wanting to say that the only person, IMO, who could possibly have had a reason, and it's a huge stretch, to purposely plant the blood, would have been MC himself. No perp would have had any reason on earth to do that and I haven't been able to come up with a single other person who might gain from doing something like that. MC MIGHT have IF he did walk away on purpose. His reason could have been to yes, protect his wife. Since I don't believe he walked away, it's really " out" there.
I've thought all along that most likely MC locked the shop door, and the house and truck, himself. I can't come up with any reason for anyone else to have done so, or even think about doing so. If he did lock his shop, he did it knowing his keys would be locked inside and he would have no way to unlock the house, truck, or shop if and when he returned home. That is one of the puzzles for me. Was he leaving a clue behind, or was he not able to think clearly?