Meaning of 'sift together'
Or maybe I'm supposed to dump all these things into the sifter at once and then sift until it's empty? 


Bingo.


  Bonus points if you can tell me if I still need to stir ingredients which I have 'sifted together' in order to mix them. 


Nope. The sifting process and subsequent stirring in of other ingredients should do the job just fine.