As rusty as the front plates on those switches are, looks like they 'been wet long time'. Not likely to look any better inside.
Shameless plug... DeOxIt. IME the hands down best contact cleaner. You have to get it online (Parts Express), and it ain't cheap. Intended for electronics with sliding or rotary potentiometers. Works on switches too. Find a hole to spray inside the switch, then soak it and work the switch thru all positions while it's wet. About the price of a 12 pack to fix it if it works, worth a shot. OTOH, you can buy a whole switch gang pretty cheap (Ebay). You can do the whole thing for the price and effort of messing with a single.
Added... Oooops, just reread and realized you're way ahead of me. Sorry, Friday night on Labor Day weekend and all that,,,🥴