"I find it just a little bit off putting to try to work out 20 frames or a third of a measure and I don't know why anybody would want to."
For the simple reason that the care to know the TRUTH about what was done in a particular video clip. If they'd rather maintain their treasured miconceptions, then they should avoid doing this!
(I will point out that the clip on this website was danced without music. Try comparing to some videos with and you will still find the general three equal divisions of the measure between the foot weightings)
On a Reverse Weave do you try to step anywere other than right on the beat.
You bet! Especially on the weave ending.
"Are you one of those who still dances to 32 bars a minute. Or are you up to date and dance 28 bars a minute. It might make a difference."
At the slower tempo, placing the steps musically rather than beginner-metronomically is even more important.
"I can't see that the second quick can be behind the beat."
Then you are extremely ignorant of proper dance technique.