There is no way Kanjuro is the traitor. Why would he allow himself be thrown 20 years into the future only to betray his liege Lord. This story can't work that way. The 9 scabbards are gated to avenge the death of Oden and if there was a traitor, lady O-Toki might have seen it and not given the prophecy.
From all indications Shinobu is the traitor. Firstly, she is not a scabbard but has had all the first hand information they have. Secondly, she was once a member of the Oniwabanshu who are nothing more than spies and she was led by Fukurokuju. Thirdly, she was quick to attack Law's crew as traitors so to deflect attention from herself. She is the only character, outside of Kitetsu and Otama, that isn't a scabbard or a part of the invited invaders. She is wholly, in her own distinct subset. Given all this, I know it is easy to label her the traitor but all other theories make no sense.
There is a sense that we are being overloaded with too much suspense with too many things happening everywhere. What happened to Sabo; what happened to Vivi; what happened to the crew all so suddenly; where is Denjiro and who is he; who is the witching hour boy; who got Law out of jail; where is Nekomamushi and who is coming with him; what happened to the fishmen crew and the German 66; who will arrive to help the Strawhats take down a 9 billion berry alliance?
On another note, looking back now, Yasuie death was the most needless in a manga that is notorious for keeping characters alive. Even if in the end, there was a coda in the cards that he added that fooled the scabbards for whatever reasons and fooled Orochi as well. I think Oda should have written it differently. There is no justifying his death.
–- Update From New Post Merge ---
On Yasuie, I feel there is something he added to that card that might have been acted upon or figured out at the last moment. It is my feeling that the scabbards have not figured it out yet and so are at a loss. That would make Yasuie's sacrifice more meaningful. There is also the possibility that Denjiro, being Kyoshiro, has arrived and acted to save the alliance from Orochi's plans. That would be a befitting entrance for the last scabbard to show up.