Interesting. I will follow this discussion.
I do not have the answers to your questions.
I am placed in northern Europe also (Denmark). I wear hood when cold, and find that very little cold water enter the ears. Therefore I do not use earplugs.
During summer I neither use earplugs, and hope it will not be a problem since the water is (relatively) warm, but would like to know if others know better?
Also, I have started using sunglasses during summer, to protect my eyes. I feel the my eyes can get quite exhausted. My sunglasses has a strap. If I also start with earplugs, it expect a tangle of straps..