Who was Darth Sidious apprentice in the ST?

It was Kylo by proxy. Sidious knew better than to groom another Skywalker just to betray him in the end, again. So he let Snoke take that bullet. And he assumed that, at worst, his granddaughter and Skywalker would automatically despise each other and the one worthy to his legacy would emerge from their conflict.

 

The ST contradicted a lot of ideas from the PT - the rule of two, the prophecy of the chosen one, the planets and alien species - these are all up in the air now.

There are lots of ways to contrive an in-universe answer if you want to, but personally, I would rather just admit that Disney threw the PT under a bus.

 

It's one of the things that the Jedi said once during the PT. Fat lot of good it did them. Everything that the Jedi said or did comes under scrutiny, or went up the air, following their downfall in the PT. It has nothing to do with the ST.

The PT "contradicted" the idea of the chosen one when Anakin turned to the dark side. Even Obi-Wan spelled it out "You were supposed to destroy the Sith, not join them." The chosen one prophecy is inherently up in the air since Qui Gon identified Anakin as being the chosen one to destroy the Sith even though they were already extinct for 1000 years. You CAN interpret Vader choosing his son over his Sith Master as eventual fulfilment of the prophecy. But, like many historical prophecies in Earth culture, there are probably dozens of instances that people can interpret of fulfilment of the Sith destroying prophecy throughout GFFA history and mythology.

 

“I have been every voice, you have ever heard, inside your head.” The implication is that Sidious had been grooming a young Ben Solo similar to how he’d groomed Anakin.

The new idea TROS brings to the table is in recontextualizing that ‘rule of two’ dynamic. The master isn’t just indoctrinating their student to continue on their legacy, but conditioning them to be the ideal vessel for their psyche. The relationship now is entirely manipulative and predatorial.

It isn’t succession, but possession. I think that fits way better with the selfish ethos of the Sith.