Hi Adi
Regarding other points.
1) You may consider it as close system.
2) Open discharge calculation is quite conservative one. You may use it to calculate force unless force value supplied by PSV vendor. I think you have two PSV with 8" & 20", in that case you have to prepare OCC load case for both of them. You may consider both case non-concurrent in case both are connected to same source.
3) No need to consider additional force for silencer.

Appreciate comment on above statement from Anindya or other expert.

Regards

Habib