Disclaimer that I am not some kind of theory or meta expert, I'm just writing down my thoughts. I also don't have a strong conclusion to this, I just wanted to put all my observations into one area.
So here's what I've pieced together so far:
Staff Thief is confirmed(?) to be a Palian Man.
Staff Thief also leaves black feathers behind everywhere he goes.
Beast of the Elderwoods was injured by some kind of giant animal, not Hassian.
I'm ASSUMING we're going to find out that Staff Thief is ALSO some kind of shapeshifter a là Beast, but since it leaves behind normal-looking feathers I don't think it's an Echo-based transformation.
Because of where we initially found the feathers (Aeroflow temple) it COULD be a black Silverwing.
However from the new Year of the Horse poster and some comments made by Nai'o in the new bulletin board quest, we now know that (although still horselike) Riffrocs have feathers.
So staff thief at this point if a shapeshifter is PROBABLY either a black Silverwing or Riffroc, but Subira said Silverwings don't typically come in that color and Silverwings aren't associated with Majiri, but Riffrocs ARE, since we now know that humans had their own horse mounts.
Other than him acting sus occasionally, Eshelon doesn't seem to be the Staff Thief, but obviously he could just be a REALLY good liar. But, the devs have pulled the "are they evil or not?" trick once before with Tamala immediately prior to this questline and I think it'd be silly if they did it twice in a row, I also would hope that they wouldn't be so OBVIOUSLY suspicious with an ACTUAL villain.
My points against him being the thief: he's using a relic like us to ward off the Echo/Beast, the staff is nowhere in sight. He helped disarm the Majordomo, the Majordomo was definitely not friends with him, he's helping us upgrade the signet rings, he's overall really helpful etc. His demeanor is a LOT like Kenyatta where he doesn't want to seem childish.
My points toward him being the thief: It's interesting how every time the player is in danger from Echo, he's suddenly there to help. Is he perhaps WAY faster than a normal Majiri would be (via shapeshifting)? Or, is the player simply not noticing him walking up to them because y'know, each time they were busy fighting Echo invading their mind or face to face with the Beast. Also he knows how to do a Flow recharging ritual and is fine with keeping things hidden from the Order. Tamala has a strong suspicion of him (yes I do trust her instincts somewhat, even though I recognize that she may just be biased against rich people, which, same).
Points in both categories: If you tell him he needs better friends he sighs and says "yes, I do."
If he IS the thief I 100% don't believe it's for evil reasons. Whoever is responsible may be misguided. Or may be misinformed. Or bound to the cartel like Reth and Zeki and forced to do their dirty work.
On a purely meta standpoint, Eshelon has a shop and is confirmed to be getting a friendship line and maybe romance in the future so for logistical reasons the devs can't throw him in jail.
I still have a theory in the back of my mind that Subira's former partner is responsible though. We don't much about him other that he was her Defender, and Subira hasn't traveled with a Defender since. I totally forget if we were told why though (and the wiki is no help) but a rogue Order member would have exactly the right amount of combat and Flow knowledge.
Random, but while rereading Subira's romance questline I unearthed that apparently Eshelon owns a vineyard (Eshe must not dislike him THAT much to be recommending his products to a Watcher lol)
