I see where you’re getting at with visions being the easy way out. But I think in this case it’s 100% warranted. As I said before, Rey says that he can’t stop seeing what he did to his father on Pasaana. He’s about to make the same mistake he made with Han by striking down Rey. By showing Han’s death before he does it, it’s like Kylo’s subconscious telling him: “Hey, no, we’ve done this before, we don’t like how it ended.”

We also make his later vision of Han not come completely out of nowhere. But if you want this vision to also not come out of nowhere, then we’d need to see flashes of Han’s death even sooner in the movie.

As for why he goes to Vader’s mask, I think it’s simply because Palpatine is a notorious liar. He wants to make sure that Vader really never was speaking to him and Palps isn’t taking credit for something real. Granted, I do see the appeal in Palpatine saying something like “I am the only voice you will ever hear in your head” earlier on and then Han’s voice is what awakens him.