Plenty of holes in this theory.
For starters, he never figured out how to get the entire room covered in blood, only a small section of the ceiling. I was thinking it was something like a large packet of blood attached to the door such that the explosion scattered it.
Second, he forgot that in earlier panels we saw others around while he ate his snack, so he can't have left - and plans was invalidated for how risky they were before, so a plan that risks being seen (or rather, not seen) not eating snacks is even more risky than a maid being busy elsewhere for a moment. Not to mention how it required the other ppl to have not eaten their desserts.
Third, 5 minutes of snacks is not enough for the ~1-2 hours of waiting for and doing 2 audiences.
Fourth, absolutely no reason for him to eat her head. Guessing he actually didn't kill her and the body is someone else's, and he is protecting her (likely helped her escape) by ensuring they can't find the real body's head.
Fifth, he would have needed to be let into the place. But they said that could only happen if she opened the door for him (sure, that's what the whole "she is his childhood friend" was about). And she can't open it anyway, because she doesn't know he wants to be let in (thick door with no sounds allowed through). Unless he agreed with her before leaving that she would open at some specific time - but that is an extra risk as she can't know if there is noone around, nor can she accurately judge time.
Sixth, when transforming, if you know her real face, there was absolutely no need to bother with that statue trick. Just transform to have her face (like the magician mentioned at the start) directly. As such there is no indication that only leo could have done all this. Except for how he ended up confessing because he promised to supply the evidence for mc's conclusion.
Finally, they say mc has no evidence. But if his suspicion is correct, the statue should have a damaged neck. Not enough as evidence, but at least it is something that when absent can disprove his theory.
ps: was surprised mc did not bring up how the entire room had been chilled and was really cold for them. It would have been used as an indicator that his theory might be right, if the prior ppl didn't have a cold room.