1) not really. Saying the fight will go the sane way implies that the speed gap is just that high.
2) don't really need to given that it's more than sufficient. However sanji showed comparable combat speed to doffy who reacted to a meteor from point blank range with ease (a MHS+ speed given the speed of said meteor)
3) first off, given the ease at which sanji dodge a surprise attack from katakuri, it's pretty clear that his COO is far from "basic".
Second, as other users have pointed out, gaara isn't stronger than sanji. He just has greater range which is easily dealt with by sanji's superior speed and mobility.
Third, gaara's MO hasn't changed much since he was a kid. At best you could argue that he has refined his sand manipulation and can control a significantly larger amount. IMHO, pretimeskip gaara was a better fighter due to being more ruthless and arguably making better use of underhanded tricks e. g. His body sand clone against rock Lee.