I can only assume that the Vader helmet footage was originally meant to be placed before Kylo arrived at Palpatine’s Fortress? Maybe he heard Vader’s voice beckon him to find the source of Snoke’s power, only for Palpatine to reveal it was him all along?
I really dislike how the quick flashbacks were edited in this movie so I would just remove all of them, and remove Kylo’s appearance during Rey’s training because it only makes things confusing.