Alright then. So that's out. Hm. Let me try a different angle. Looking at the note to Franziska, I see a set of numbers. They can't possibly correspond to letters so I'm guessing the numbers are code for something in Larry's letter.
Potentially. However, I cannot make a conclusive red for you on the matter without another theory (one that can explain the circumstances behind both deaths and/or the locked room mystery of the study).
Well let's say the paper was used as correspondence between Larry and Franziska. That means they either were lovers or they were working together on that corrupt prosecutor case. My guess is the latter but whatever. If neither of them are accomplices, then the accomplice was someone who's alive. Since Klavier was seemingly in the file room the whole time, I'll rule him out. That means that Edgeworth was the killer but Phoenix was the accomplice. How else could Edgeworth have gotten the wine cellar key?
That theory sounds plausible. Although it should be noted that if Edgeworth is the culprit, there is also the possibility he could have lied about having the wine cellar key. Same goes for Phoenix and the dining room key if he's the culprit or Klavier being in the file room the entire time if he's the culprit. There is also the matter of the first incident and how it correlates with the rest of the story.
Okay then. The first incident was a fake suicide by Larry. He took enough sleeping pills that would result in cardiac arrest but not enough that he would actually die. As for the blood, he took some of the shards and cut himself so that people would think he was killed at first.
Then the culprit poisoned Larry’s drink to make him fall unconscious? After all, we saw some broken shards in the library. Then, when the killer realized their first plan failed, they killed him in the bathroom.
Then who we saw in that first incident wasn’t Larry Butz. Gavin was knocked out. Possibly with the sleeping pills. Perhaps he was placed in Larry’s clothes.
Hm. Well let’s try this. The first incident was Larry’s actual death. The second Larry that Payne saw was someone in disguise. Likely the accomplice. You said that no unobvious disguises could be used but perhaps comparing the two bodies could reveal a difference. After the incident, the culprit moved Larry’s body to the bathroom in question.
Who said Larry died in that bathroom? The culprit killed him somewhere else and transported his body to that bathroom. As for the other thing, he was found dead in the other bathroom, but not necessarily by Payne. Perhaps the accomplice helped transport the body.