He said Inuarashi and Nekomamushi can't be traitors because they proved loyalty. So did Kanjuro.
If you read the spoilers, the reason why Kinemon couldn't believe him initially was because he had proven loyalty.
The person who made that video got it right, granted, their reasoning is not correct. In essence, he made a good guess.
There was no evidence or much hint.
He said Kanjuro telling Shinobu to calm down = His wanting Law to get cut? Looks more like he's actually trying to calm her down. That's what an ally does. Calms people down and stops them from acting irrational.
He says Hawkins using Kairoseki nails against Law is proof Kanjuro was the culprit. Hawkins fights with nails in general. Why wouldn't he have Kairoseki nails to fight DF users to begin with?
His reasoning is incorrect, there really was no hint or evidence.
It was literally Oda choosing to making Kanjuro the culprit which
1. His resemblance to an artist doesn't really indicate he was acting. There's a lot of reference to Kabuki.
2. The birds do not necessarily indicate anything other than we see birds. Do the birds look like drawings? Not to my recollection.
3. As you said, Kinemon and Kanjuro knoew. Therefore, we could have said Kinemon was the traitor, but is there evidence? No, and the same case can be said for Kanjuro. There was really nothing outstanding.
4. A Kurozumi got access to Momo's face. Anyone who can take on the apperance of others can infiltrate anywhere. Someone who can change identity doesn't exactly need an insider to let them infiltrate. Could just appear as a kid and enter the Kozuki family. It's easy is my point.
5. That's not proof. Anyone could have been the spy if Oda wanted it to be.
I agree it was hard to figure out , i initally did feel it was kanjuro but after the flashback it was hard to tell who it was.
I had kiku in my option as well. Only thing she couldn't be traitor was she didn't raizo's location.
Only thing i could tell it was kanjuro because only he knew of raizo , isnt it weird what led jack back to Zou easily . Kinemon proved his loyalty , i even thought him to be traitor as well but he tested his loyalty well.