for most wheels an easy and effective thing you can do is to isolate the areas of the arch above the motor along the seal of the wheel body with something like a butyl braid/cord/tape and isolate the area under the handle with a very strong and sticky tape, something like filament tape (tape reinforced with fiberglass) applied along the seams of the two halves of the wheel body under the handle (also in other places susceptible to water leaking in if possible)

a general next step is to seal the controller, lights, buttons and all other connectors and things that look dangerous inside with a sealant

there may also be some vulnerable components/areas specific to your wheel, for that you may have to search some tutorials on youtube/forumse etc

there are likely local EV shops near you that are doing some sort of waterproofing for a fee, they also can help seal the bearings and the motor and some other useful stuff

no, not really

i ride with a regular summer tire in the rain a lot and never experienced issues, the traction is reduced, yes, so you probably shouldn’t try to do pedal-scraping turns at high speeds, but other than that you’re fine

from the outside perspective i guess it would seem that an EUC is very slippery and unstable, and to some degree that is true, but with a proper tire you can ride on any surface just fine, for example there are winter tires with a lot of threading and spikes that allow you to ride on basically pure ice without any issues)