Yeah, but at least Helseth was shameless about it and was one willingly, because it gave him power and influence. Plus, he was willing to kill for what he wanted. Elisif just seems too much like a child to really know what's going on.
And Riften svcks, no matter how one slices it.
Helseth was very conniving- he was out for power anywhere, anyway, anytime. He is perfectly content with murdering, backstabbing, lying, and if necessary, flattering his way to power. Note his treatment of the Nerevarine: he knows full well the Nerevarine is a massive threat, so he tries to kill him. When that backfires, he completely and baldly admits it to your face, then has the gall to say "yeah, tough luck. Don't want it to happen any more? Here, go do this stuff for me." THEN puts his own mother in danger (possibly) to kill you.
I believe if Helseth ever saw the ability to usurp the throne of the Empire, he would seize it in a heartbeat and never look back.
Does Elisif have that same ambition and cunning, maybe not. But it's my belief she may actually have it, and know that the Empire is her best bet to get revenge against Ulfric. When that backfires (IF it backfires) she sides immediately with Ulfric.
I wouldn't be surprised if that's the way Bethesda is going to clear it up- if Ulfric wins, she assassinates him and gets put in charge. If the Empire wins, she's in charge anyways.