Got it set up and it works perfectly. It’s a very clever solution that I never would have come up with myself.
However, the problem is the lag - if a lap is entered incorrectly, the checkbox won’t change the values on the sheet for about 10 seconds, meaning that when the student name is tapped to remove the lap they will actually continue to increment laps during the lag. When the log value changes to TRUE, subsequent taps will remove laps. So while it works, it won’t be practical in the field as teachers may lose track of what the lap tally is actually supposed to be with the tally going up and down.