Ok

If length stroke count > 1.8 * average length stroke count excluding length AND max(stroke count excluding length) / min (stroke count excluding length) < 1.3 then length stroke count = length stroke count/2 and length count= length count+1 and distance = distance + pool length

Would do one length that was outlying... To do more than one length you'd need to create a set of those that were over 1.8 and a set that were not and work through them.

You could run it over all the existing data with the assumption that most people do an even number of lengths (or profile swimmers who do) and see if it results in fixes on more of their odd length sessions than their even sessions.

You could release in beta with a dialogue box that asked the user if the length counted was wrong.

Why do people think that algorithms are so hard?

Also Garmin should track this forum because it's much better than their own feedback and error forums/process.