By the way I also think that Rey striking Finn with lightning is too overkill, she’s supposed to be pushing him away to keep him out of it, but ends up hurling him by pushing with great force, I think that’s enough to show she’s lost control. At that point Rey is supposed to believe she’s doing the righteous thing by fighting Kylo, trying to prove him wrong by refusing to join him, not realizing her frenzied state is proving the opposite until Leia’s sacrifice makes her snap out of it. I think literally electrocuting Finn with a bolt of lightning would be enough to make her snap out of it in a similar manner.
As for Palpatine’s lines, as it stands now he’s already pushing for that deceit that Rey will inherit his power, when he says “only you have the power to help them” he’s saying only she can take over him, inheriting his physical power AND the power of his position: it will give her his power to command the fleet, which would be the quickest way of saving her friends who are currently being shot down by said fleet.
Of course it’s all a lie, as the spirits of all the Sith would overtake her. I do like this concept of “All the Sith”, Palpatine has always been the representation of pure evil, and this explanation actually builds upon his character. It’s interesting to think we never got to see whoever Palpatine truly was before killing Plagueis, as it turns out the Palpatine we saw in the prequels was always a conglomerate of every Sith that was able to overtake his Master. Of course in classic TRoS fashion, this concept contradicts the idea that Rey’s power stems form being Palpatine’s grandaughter, since Palpatine’s power is not genetic or even physical at all, unless his offspring was somehow able to inherit the genes of every spirit in his body, and that’s not even including the mess of making his son a clone. This parental relation is also the reason they need to have everyone call him “Palpatine” instead of Darth Sidious (which Luke called him in TLJ) muddling the concept further. It would make more sense for him to use his Sith name instead of the name of the person that was overtaken by all the Sith.
Kylo’s line about killing Palpatine comes directly after he tells Rey they’re a Dyad in the Force, and he specifically says they will kill him “together”. I think it’s simple to put two and two together and imagine Kylo would think they can defeat Palpatine because they’re a dyad.