Okay so after going back and playing through all Kas's content this is what I can come up with that I could remember/infer. I'm putting it in a spoiler cause there are spoliers to Kas's plan for whoever wants to find and read it themselves.
To answer your questions more directly:
Yes the champion is trying to find a way to redeem Kasyrra and/or help her achieve her goal without her needing to do thing her way or take their soul.
She's not trying to corrupt them but she wants their soul. I explained her plan better in the spoiler.
I haven't found much of the companion's opinions on the matter but based on one interaction in the Winter City I think they would be confused and upset depending on the character since they see Kasyrra as the big bad demon lady trying to ruin/takeover the world.
Kas's reasoning is more selfish than noble, although it's not quite the malicious evil bad guy it's set up to be at the beginning. Again more details in the spoiler.
I get the impression that Kasyrra knows exactly what's up with the gods in Savarra that the champion figured out from Nareva in the frostmander quest but she's using their position and inability or reluctance to interfere in the mortal realm or whatever to keep them out of the equation until she gets what she wants or needs to get rid of them. I wouldn't be surprised if she was at the very least pretty close to them in terms of power based on how blasé she is towards them but we haven't been given much in the way or power rankings so that's still up in the air.
Kasyrra is purposefully leading the champion on a wild goose chase to make them become stronger to strengthen their soul until it will work for her plan. Her interest in the champion stems primarily from the direct role they could play in helping her to achieve a goal that she obsesses over due to her demonic nature.
Her primary objective centers around the MC and not corrupting the world so she sees it as more of a game that keeps her entertained and furthers her agenda than the champion ruining her plans, and she picks up more romantic interest in them along the way. It's not undoing her work since she's putting it all there just to prep the champion''s soul so she can take it and use it when it's ready.