First off, I couldn't care less if these things are retcons. The two(actually three) biggest plot points in the OT were pretty big retcons.
That would be the easy answer for Rey. For Rian he felt for Rey at that point, the hardest thing she could hear, is that she a nobody. That's literally her worst fear. And that's great. But she makes peace with, she accepts that. So moving into the next film, where's her internal conflict coming from? Luke's it's because he know now that Vader is his father, and he may be left with no choice but to kill him. So what's Rey's? If this story is about Rey forging her own path, despite who she is. What can you do to derail that? JJ and Terrio's answer is to pull the ground from under feet. She's just got herself established, she's starting feel confident despite being a nobody. And bam, a curve ball comes sailing in and smacks her aside the head......she's Darth Sidious's granddaughter. All that self confidence she gained, comes crumbling down in an instant. Now is the best way to continue her story? I'm not going say. I kinda like it. I see where JJ and Terrio are coming from. It was obviously a difficult decision to make for them. Because they definitely wrote a version(s) where Rey was a nobody, or so claims Daisy Ridley.
Yes Palpatine is dead. And everyone in Star Wars, and fantasy, and mythology, and pulpy serial stories remains dead.........NOT! If ROTJ had been the end of the story it would be one thing. But it's not. And no, I don't believe you need several hints at his existence. With only three films, there's very little you could put in them that wouldn't let the whole cat out of the bag. For me this is more then enough of a set up, and pay off. For me.
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I'm not even talking about Plagueis. Anakin was created to bring balance. Palpatine was the one causing the imbalance. So Anakin was creat to counter Palpatine. It's no mistake that the moment that Palpatine begins to put into motion his master plan to take control of the galaxy, the Force serves of the Chosen One on silver platter for the Jedi. Palpatine is aware of the prophecy, and he knows he must work to undo it.
Kylo never seeks validation or approval in TROS. He's very sure of himself.
As far as your other points. The lightsaber calling to Rey, i.e. the Force is calling to Rey, is a set up and then paid off at the end of TFA, when accepts the call of the Force. I mean it's very clear that Rey has been chosen by the Force. She's had dreams of Ahch-To, and the Jedi library. And in TROS we see why, she's the other half a dyad, a Palpatine, and she's been destined to restore the balance that Anakin entially brought to the Force. Though one could way there's a second pay off when she takes on the Skywalker name, as the Force is calling to her with the Skywalker lightsaber. That's the long answer. Short answer. The Force was calling to her.
And Finn, tis true, they could have done more with his relationship with the First Order. But then again, I get why they wanted to focus on the main characters. They ended up repeating the same mistake ROTJ did. They get so focused on the main characters, your supporting characters just kinda end up existing, but not much else.