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?
Absolutamente.
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?
Novamente, absolutamente. Definitivamente torna as coisas mais fáceis.
Should I replace the loose fill insulation with unfaced batts and fill in any small areas with the old loose fill?
Não é necessário se ultrapassar o espaço condicionado, mas não seria um problema.
Does all of this sound kosher / is there anything else I should be considering?
A viga é apoiada pelas ripas e gesso abaixo na parte inferior, e a sub-base no topo, então não será necessária nenhuma ponte.