There are other implications than sanitary for chilling. Hop character, DMS, cold break, haze, etc I am sure there are others. Also, chilling in the corney creates a vacuum, as you alluded to, but opening the PRV is just going to draw "dirty" air into the corny. The ground water gets pretty cold this time of year. I am pretty sure mine is like 44 or so right now, although I haven't checked at my house. That is what it comes out at at the still. Your wort cools down very quickly with a chiller.
This.
Why even bother with a chiller at all?
I want to get a good, quick chill and pitch. I really don't like the idea of leaving wort laying around to cool. I know that some people do that but I prefer to get it cool and go. I was at a brewday at another brewers house once in the winter and there were a bunch of us there drinking and after the boil he put the brewpot in a snow bank and just kept packing snow around it. It was fine and he said the beer (a kolsch,
IIRC) came out nicely. But I wouldn't rely on that... chill it and be done.