Yes of yourse, several users switched to more stable wheels.
There are exactly two things which make the LT versions cheaper than the others - the plastic bottom bracket excenter and the plastic wheels - now you know.
18" wheels are much sturdier than the wobbly plastic LT wheels,
their disadvantage is less comfort (because matching tyres are narrower) and there is not much tyre choice available in size 355.
Not mandatory as long as your brake rotors and freewheel are in good condition.
(But you need some tools and should know a few tricks to mount/unmount these parts.)
Correct; 16" and 18" fenders have different sizes.
I’d say it does anyway because all the force you’re wasting now in
deforming of the plastic rims
can be used in future to propel you forward.
Regarding test rides:
At this map there can be found a least two Strida owners within 80 kms of your location,
three more at the area of Brussels,
perhaps you should try and ask Bill:
https://stridacanada.ca/map-strida-canada/