The pcb should be sporadically dipped in water, for about 1 second.Questions:Is there some water resistant resin....
I think the PCB does not produce significant heat, so I guess this will be not a problem.
The can you linked to is good for corrosion protection, but not for overall waterproofing.
What conformal coating will protect against is condensing humidity. It is good at that.
Yes that is the point of conformal coatings. It is called probe through. It is meant for servicing. Once serviced you need a quick spray to it again, unless the coating is self healing.The coating is ment for a board that is mounted in a box, not a free board that is handled.