Depending on your airlock you can get suck back from the temp drop and find your airlock nearly empty with its contents in your beer. S-lock... No worries
Yep, the S lock, as long as it isn't over filled (I'd do slightly underfilled) shouldn't drop any liquid into the beer. The suck back is one of the reasons I don't cold crash until the beer is kegged, since it has to be sucking in oxygen as well. The o2 exposure probably isn't much, but almost certainly more than I get by crashing in the keg under co2 pressure.
I have absolutely no data or taste testing to show what, if any impact long term though.