Hello BSA
I think the problem is originally from the low strength of aluminum… As you know, the thickness of rear-tube is very small. The length of thread is quite short 
The thread can be easily spoiled by a mere force. So I agree Ming’s advice, “Don’t change the pin position often.”
To analyze this mechanical matter, a recommendable approach is to choose knowledges of Static Friction. Before the seat-moulding slips actually (with an actual velocity), the seat-pin would take a small portion of weight. Physically : WEIGHT = Big frictional force of moulding + Small shear force on seat-pin 
I have been waiting for a Height Adjustable Seat for a long time. I wanted it because lower seat is safe for running down and higher seat is helpful for climbing up. And now Ming’s solution is satisfactory. (but I didn’t buy it yet 
)
Although I am not in the Strida business, your demand seems not usual… not acceptable for many of Strida riders. Sorry to say this 
Binch