Ventress attacks without stealth? I wouldn't say that, I would strongly disagree. She is an Sith assassin remember and on numerous occasions has been dispatched to deal with her enemies via stealth. For example when she rescued Gunray, when she used the Clone Trooper (Slick) as an informant on Christophsis, when she acted as a spy on Kamino, when she tricked Jabba in to thinking the Jedi had kindapped Rotta, when she used Savage as a pawn etc etc. Stealth is practically her middle name, along with Nightsister and assassin and subterfuge which are synonymous with stealth.
And also the fact that as we see in the trailers, it is her that Ashoka confronts. Seems a very unlikely coincidence. And if it is Sidious, who killed the Clone Troopers? I can't imagine Sidious getting out his dark side get up and sneaking around a military base to kill some troopers, looks like the work of Ventress.
And remember that Sidious is the Chancellor, and Letta said it was a Jedi. Letta would be able to recognise the Supreme Chancellor of the Republic I'm sure, and not mistake him for a Jedi. But then again, Ventress, or whoever it was, must have a Jedi accomplice. Otherwise how would Letta have a name? Come to think of it, why would this 'Jedi' give her a name at all?
I also doubt its the Barriss Offee because she dies in Order 66 (cut scene however) which means she's with the Republic till the end. Then again that could easily be retconned as it never made an appearance in the films, but she seems very light side, pro Jedi to me...
Oh, my mistake. But Sidious is a Dark Jedi and could of made Letta think he was a Jedi (Though he is a sith lord.)
And I just had thoughts of a chance of it being General Skywalker possibly, because he has used Force Choke before as seen in a episode a while back he used Force Choke on Gunray to get Gunray to tell him something which I forgot. Though, I don't know why he would Force Choke Letta, but who knows.