Maybe it's a bit of wishful thinking and it doesn't redeem the mess up to this point, but it could retrospectively tie together a lot of the loose ends going about, while trying to make Sylvanas not look stupid, while reconciling the current version, with the old one.
It'd largely be revealed through exposition during her interrogation. Not the most creative way to do it, but there is a lot of ground to cover and it makes sense she'd spill the beans since she turned on the Jailer anyway. (Kind of relying on the cata and onward retcons being just someone blabbering in interviews, if it has to be kept then she """""served""""", with a great deal of autonomy doing largely her own thing, until BfA) She'd have a relatively calm and confident aura of a natural leader, with a rich vocabulary and wide range of expressions to remind the audience she is meant to be both very intelligent and charismatic.
After Gorribal has been cleansed with all the artifact weapons it's living essence made it¨s way into the shadowlands and broke the arbiter. (It's red and fits the timeframe) This lead Sire Denathrius to initiate the soul drought over the 4 Covenant's realms and had the Jailer's plan gone as he imagined it then he'd slowly choke them out, until they'd be too weak to stop him from claiming the sigils, without the heroes of Azeroth being any wiser to it, until he basically completed the scheme and dominated the whole universe. So she needed something massive to throw a spanner into the machinery of death, while still feigning allegience. Noting that had she come to the other leaders, who are largely in the know regarding Bolvar they'd ahve waved her off as vengeful and holding onto grudges, while not really paying mind to what she said. She needed a massive influx of souls to force the issue and Teldrassilw as prime target for 2 reasons. Kaldorei are vulnerable to swift decisive attacks due to their habitual aversion to building fortifications making it by far the most straight forward and she really wanted to get back at Greymane for what he did in Stormheim. After that the huge conflict was inevitable and things were set into motion. She'd also have convinced him that he could use the mortal champions of Azeroth, who are mighty and stuff since he needed to act sooner than he'd like. She made it a war they could fight, one they could win, even if they had to be united against her and more importantly followed the path she set for us. Noting that Anduin is an idealistic idiot, who can't take a hint to save his life so Zovaal personally oversaw him getting fitted with a perfected domination armor. The Jailer couldn't be slain so long as he was apart from his sigil and within the Maw, while he could feasibly be subdued it'd just be pushing the same issue down onto the next generations, while her Forsaken rot in there with him through no fault of their own, for countless years. When asked about why rebel now? She'd say that she had used Zovaal's extraordinary pettiness to save them from being dominated, because she figured he'd let them have her and thusly not dominate them, in order to punish her. She didn't expect him to return her the shard, altho she suspected he had it, ever since the Edge of Night.
When asked why they shouldn't just end her she'd flatly reply they'd be doing her a favor. Sparing her the inevitable eternal torment and humiliation, when Zovaal claims his prize and doms the whole universe.