First question, no. If you are force carbing, then you are injecting CO2 into solution and do not need the yeast to produce additional CO2 for you.
Second question, yes. If you are not force carbing, then you treat the keg as if it's a big bottle. You add the necessary amount of priming sugar and then set the keg aside for two weeks (or more) to let the yeast carbonate the beer.