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I also have come in late to the debate, given flexure was an issue I tried to sort for years I have had some experience in this.
Firstly a point perhaps missed is from exposure to exposure you have the AVERAGE for each exposure.... Ie your 10 min sub is the average for that 10 mins then the next sub is the AVERAGE for that next 10 mins, there is no magical jump.
Secondly so why do my stars appear round.... Again laws of averages, you have also to allow for atmospheric distortion, corrections within the guiding in fact any variability which will smear your image. IF the air was perfect and NO movement whatsoever and you had a perfect mount you would notice the trailing ( out of roundness) more.
Much of the elongation at times can be caused by overcorrecting in the guiding, ie overshooting left...right....left etc
So in conclusion, it's most likely flexure causing the drifting.... Try a 40 min sub to allow for more movement and this will be confirmed.
All you can do in the meantime is make all your connections as rigid as possible. Wherever you have a separate guidescope there is some flexure, it just comes down to how much.
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