Hmmm... the fact that the dealer has installed a 'carb heater' makes the guessing more difficult..
And there is no point now in a lot of guessing about all the ups and downs in the machines recent history.
I think you are over-thinking this by guessing at a 'soft-seize'.
(I doubt it had a seizure)
If I beleived it might have had a seizure of any sort I would simply pop off the head (not the cylinder) and push the piston to the bottom and take a good look at the cylinder walls with a good light.
On an air cooled 2 stroke that is soooo easy that you should just do it.
As long as the cylinder is smooth and unscratched/scuffed... then it didn't seize.
If you see scuffs, then take the cylinder off and you can get a better look.
And Yes... I am sure you are making a Royal pain in the ass of yourself to the dealer
... and your amateur questions are surely annoying them.
Consider that it is not the responsibility to teach you "engines 101".
Fixing machines and pleasing a customer is not an easy task.
If the airfilter/airbox/carb heater is the way you are going to leave it, and you are unsatisfied with the performance, and the spark plug is dark brown/black... Then you can go ahead and try one step smaller main jet at a time to see if it runs better... but examine the spark plug color each time.