you can prime with sugar in the keg, some folks say to use about 2/3 as much but I don't know that it really matters a huge amount.
If you purge the keg well with co2 I am not convinced that priming gains you anything over force carbing except space in your keg fridge.
if you have enough kegs there is no reason why it can't stay in the keg for a year or more. I only own three and one has water in it so I only have the 2 for beer, I don't age things in the keg.