I was under the impression he(Sanji) predicted both shots.
Nope - the priest didn't shoot until Katakuri hit him, and his shot went wide.
I don't see that as a weakness. Because Sanji is somewhat a prodigy on Kenbun, he would dodge anything that would come to him, besides that's what the usual power does, attempt an attack.
Not necessarily - Observation doesn't let your body do anything out of the ordinary. If Sanji was forced to go into Katakuri's line of fire in order to avoid the priest, then he'd be dead.
For applicated simplicity here, let say that normal users attempt movement, potato attempts situations. In that regard, his sight is an outstanding feature of him, and does not have a weakness because he sees the situations where you're going to make a movement… Haki was made to stand above devil fruits after all, and still Oda gives him a Logia. Cracker was fine, hardest Buso and great skill over his power, potato is just overwhelming... makes Jozu and Marco feel like Buggy and Galdino.
Yes, it's very strong but it does not work well with everything. Katakuri might be able to see that movement, but he will not see what else is going on around that movement. Thus, he can deal with that movement as best he can, but will be unable to anticipate another party jumping in and sabotaging him. Like when he saw Luffy attacking the portrait of Carmel - he either failed to see Brook doing it later, or he mistook Brook for Luffy and assumed he prevented the movement when he went after Luffy.
And Logia doesn't equal strength. We've met Logia users like Caribou, Monet, and Caesar who were total patsies compared to guys like Doflamingo and Jack. Plus, Haki can be a complement to Devil Fruits, like with Cracker. I don't see how the two need to be separate. Marco and Jozu both have powerful Devil Fruits + Haki.