On the Any% No OOB No Ride Skip run, all of the rides have a story attached to them apart from the Merry-Go-Round. You're able to spam left mouse when the ride is slowing down to leave it slightly early. On all the other rides you have to wait until they're completely finished.

Is it okay to leave the Merry-Go-Round ride while it's slowing down, or is the rule that you need to wait for the ride to come to a complete stop?

As long as you rode the ride you can leave it in that instance