Some random thoughts:
- At some point in 0.4 Bitey gets to know that Frank didn't mention anything about Aloys going out at night because they are friends and he's used to covering him every time he leaves at night. I think it's very likely Aloys would at the very least do the same for his friend, avoid mentioning that he leaves the room at night sometimes, to make up for the favors. This would explain why Aloys never mentions anything about Frank leaving at night
- Frank is one of the characters that could easily slip through night without Ben noticing at least who he is. Ben is able to recognize that some things are going on outside his room but he can only confirm if it's Santiago or not, since they live very close and he's used to hearing him after his night shift. But for Frank it'd be a different story since he lives in the monks dormitory. Ben says "From both the detectives and Giorgio sneaking around, to the hunters doing their shifts and monks being rowdy in their cells". This means that there's enough activity outside (even at the monks' cells) for him to make out what's going on around there. The other character that would leave at night without Ben noticing would be Aloys, but I think we can all agree at this point he's not the culprit. Then we have Remigio but to me him being the culprit would make all the murders very easy for him to commit and it wouldn't make much sense.
- If Remigio's testimony is true, then Frank fits the "human face" description. He also may fit the clue of the dark hair found in Oscar's room.
- Even though we don't know what connection there is between Frank and Remigio, we know that they do same a connection through Aloys. This means that there's plenty of scenarios for him to steal Santiago's borrowed keys from Remigio.
- I don't think the whole book thing for the 3rd murder would be irrelevant and that the only answer is just Remigio went through the window, killed him and left. The whole window clue has to be a red herring.
- I already mentioned in the main post but Frank may have something related with the descendants. Since he got attracted to Alessia's potion aswell, which was meant for Lucusti in the first place.
- I think by now Bitey is suspecting Frank. Besided the descendant-luring potion we also have that last scene in 0.5 where he gets asked about one of his previous cases and he especifically picks Alessia's first case. Possibly to find a reaction on Frank or Aloys, since he finds some similarities between both cases.