I don't like the idea of using silicon sealer around electric terminals, it contains acetic acid which is, well, an acid. Not a good idea. I use Silicon GREASE.
Ah yes, I recall now having tried that some years ago. The screw heads oxidized. Being brass they turned green and were rendered useless as they crumbled when I finally was able to get a screwdriver through the silly-cone down to the head of the screw. I should have known better being in the construction business. Over the many years I have seen silicone caulking do some ugly things when placed against copper or other such building materials.
I wonder if dripping some wax into the openings might be a better solution ? I think I tried hot glue at one point. It removes easier when placed in the fridge for fifteen minutes.
Biko switch drives have rubber covers that have nubs which fit into the holes where the screws are. Even with this seemingly bullet proof design, moisture finds it's way in.
It's not always water getting in as you might think. Condensation forms inside the switch drives all on it's own. Maybe miniature umbrellas would prevent not only direct water entry, from above only, but shade the drives from the sun, thus preventing condensation.
We're overthinking this issue I fear. On my own railway I removed all of the switch drives and control the turnouts with manual labor. It's foolproof.....And believe me I know the fool personally.