OR "Trying to make sense of her actions up until now"
This isn't Sylvanas fan cope or cry for redemption, this is me trying to find a reasonable explanation for everything that's been happening with her until recently. And after the latest abomination of a cinematic I've come to a conclusion. And as much as it pains me to say it, I think the Jailer has been compelling Sylvanas to do his bidding the same way Arthas had done years ago.
My theory assumes a few things:
1. Souls that had direct contact with the Frostmourne and the Helm of domination are vulnerable to Jailer's attacks
2. Jailer needs a lot of anima to exert his powers.
End of WOTLK:
Sylvanas kills herself atop the Icecrown. Her presesence becomes known to the Jailer who, desperate to break free, seizes the opportunity to reclaim the former slave of the Frostmourne and gain a potentially useful asset. He hijacks her soul and brings her to the Maw where he binds her soul to his will. He sends his sleeper agent back to Azeroth. He doesn't have enough anima to fully control her there but he's playing the long game. He knows that in time she'll prove useful.
Just to point it out, when I say mind-control I think of the way Lich King controlled his death knights and the like, that is the voice in your head compelling you to do things, you're fully aware of what you're doing but can't stop it. So not exactly a mindless zombie.
CATA and onwards
Sylvanas mostly does her thing, the Jailer's voice usually not loud enough to be able to control her. Perhaps she's trying to find info on him behind the scenes.
LEGION
Jailer's influence grows, he sends Muelzhala to name Sylvanas the warchief, the time's when she'll be of use is fast approaching. This is why it's surprising to Sylvanas. She wasn't in on the plan. (Perhaps the deal with Eyir in Broken Isles wasn't for getting immortality but to try to turn Jailer's forces against him, but that's just baseless speculation and retcon territory)
THE ARBITER BREAKS
With the souls pouring into the Maw, using his powers to control Sylvanas becomes trivial to the Jailer. She's no longer able to fight his voice. He sends her to spread death and she does.
BFA happens. SHADOWLANDS trailer happens. Sylvanas' choice cinematic happens and we're here. She acts so weird in this cinematic because she's been fighting against his control the entire time.
That's about it.