Finishing Attic Floor (Joists Blocking / Insulation)
Can I screw the blocking instead of nailing it as I'm a little worried the nailing might cause cracking in the plaster ceiling (with wood lath) below?


Absolutely.


  Is it okay to sister a 2x6 board to the current 2x6 board along the front and back perimeter of the finished area to support the ends of the new subfloor since I'll be cutting the current subfloor back to the edge of the current 2x6's? Can I use screws instead of nails?


Again, absolutely.  Definitely makes things easier.


  Should I replace the loose fill insulation with unfaced batts and fill in any small areas with the old loose fill?


Not necessary if over conditioned space, but would not be a problem.


  Does all of this sound kosher / is there anything else I should be considering?


The joist are braced by the lath and plaster below on the bottom , and the subfloor on the top so no bridging will be needed.