messed around with 100c backups some more and found out that if you fail the slide and miss a bunch of coins, it is faster both in 70 and 120 to keep going on the slide and get the rest of the coins, then leave the building and do 100c with reds.
in 70 this actually saves about 21 seconds over dying and reentering when you miss the coins, but in 120 its only about 8 seconds faster because after 100c + reds you have to do penguin race on its own.
edit: worked on the route some more, dont have a recording but if you think of the red coins in order of how they are normally collected in red coin star, once you exit the building you grab red 4 and 5 then warp, do the slidekick for red 7 but then turn around and go for red 2 then 3, then you go across the bridge with the snowmen and get 8 and the star. this is ~6 seconds faster than what i did in the video making a total of 27 seconds time save in 70, and 14 seconds in 120.