An island long forsaken looms in the distance, and a lonely witch awaits you by the shore...
She said READ. CAREFULLY.
When The Ravens Cry
The Aviarist is the deputy, being the only 'private investigator' in the story. (This logic kinda has a flaw, it seems like the Aviarist's coming to the island was a rare event so it doesn't make sense to call someone 'my deputy' if they don't come often, but as Monarch and Hunter are related in authority maybe Hunter was informed about this.)
It is strange to me that an ordinary boatman can go to the island in 8 hours while big, presumably mechanical ferries can only do it in like 7 hours, I'd think the speed increase would be more, so maybe the detour was not noticed or usable by the ferry. Considering the ferry was also on a schedule, this means his boat trip was even faster than a ferry, making him arrive earlier.
Also, his section ends with "I headed to the town hall", which means he could have died on the way there.
While I suspected the Monarch for seeming to know too much about the murder weapon (how does the Monarch conclude blade and slicing when the Hunter looked at the bodies and came up with nothing?) the motive doesn't make any plausible sense to me, sending a letter to his son and then killing him.
So, the culprit is the Hunter. This is mainly because "no narrators are lying" effectively, whose investigation results are incredibly vague. There is also a statement that doesn't seem to coincide with any facts at all: that the body was reported by someone at all. The Jester, in his side, was woken up by the Hunter, so he couldn't have reported it, so that implies someone who was reported but never interrogated.
The method seems overly simple, which probably means this whole thing is incorrect: He just walked out of the alley, the one mentioned by Jester. He just stole Aviarist's cap. Presumably the ordinary local just died to remove a witness.
Ah, dear friend, such a pleasure to hear your charming voice! You seem to be confused, however, so allow me to alleviate your predicament. I am in no way related to the army, even more so to lieutenant Dark, and clearly I am not his deputy. Also I believe none of us died yet. By us I mean the four narrators as you might have surmised. Oh and before we part ways yet again, the witch has kindly asked me to deliver you a message of sorts, an advice to ease your struggles.
She said READ. CAREFULLY.
By the way, the murder method you presented will also be a matter of doubt in the future, The bodies were in a rather gruesome state when they were found, moreover, two people died at once. It is true that Jester only kept watch from one side, but It wouldn't be possible to commit the murder by ways you described (and any similar ways), without people on the Blackfeather street noticing. It would take too much time and attract too much attention.