I cant speak to any specifics of Merlins...
Are you suggesting that the boat stands being set in sand resulted the sagging?
While I can see how that might happen, those dots are pretty far apart and difficult to connect. I can conceive of this happening if you had a flat spot / indent in the hull from the stands slipping and 'too much' weight was resting on a single
stand which was on / near a stringer... if the stringer moves the everything attached to that stringer will also move but that would, I think, result a shift port to starboard not fore to aft.
Do you have pictures (ideally before and after)?
Sorry I cant directly answer your question.