I've done this once with second runnings with a corny keg. I filled the keg right after flame out, then placed into the cooler. Once chilled I transfered to carboy for fermentation. Had no negative effects on keg or beer. The pressure drop from contraction is so minimal the keg and valves are unaffected.
I guess if there was say 50% head space then the ...
Why not build a hose that goes from the Sanke fitting to the liquid side of your Corny? I'd use CO2 at the lowest pressure that you can to get the liquid to move. Otherwise, use a growler filler with a tube that is long enough to reach the bottom of your corny keg and treat the corny as a growler. When you get home, connect as usual.
Yes. There's little difference in time if any when you're talking about 5-15 gallons.
Basically CO2 absorption would need to be greater than the volume your regulator to serve for there to be issue. Tank depth also plays a roll, but has very little effect at homebrew volumes.
There will be no noticable difference.
If you were carbing up 5 BBLs or greater then you would not just apply top pressure you would apply pressure through a carbonation stone to do it in a reasonable time frame.
If you have the sanke valve you can attach the gas to the gas line for top down slow force carb which helps fining. Or to the liquid line for co2 injection for bottom up. Or both lines, but this will basically just be a top down carb anyways.