90 psi would be the top end for pressure for champagne, the low end
would be 60.

5 galloon corny kegs should go well over 100 lbs of pressure max, I
think mine have 125 stamped on the side. The popup valve on top bleeds
pressure at 90 I think. I am bettign the 3 gallon kegs are similar.

What I would worry about is makign sure you have a regulator that goes
high enough. Most only go to 60 lbs I think.

The acid will not hurt the keg.

Make sure the winei s cold when you carbonate it, as cold as possible,
then you will have a nice sparkle at a lower pressure. What you are
looking for is "volumes of CO2", not so much PSI. And gas is more
soluble in cold liquids than it is in warm ones.