I'd be surprised if the plastic knobs fall apart from bring exposed to the elements.
On mine, I left that little bit of red locker residue on them. They are never going to vibrate loose with that stuff on there, but are still not hard to remove by hand. YMMV
As far as tightening them goes, just a few hand turns until snug worked for me. No need to try and run the bolt in as far as humanly possible, plenty of purchase with a few light turns. Again, YMMV
As someone earlier mentioned, we don't really need any bolts at all to keep the seat in place, anyway. Cheers.