If the filling of the gravity sewer is due to the backwater effect from the wet well Pump Station A, then the Gradually Varied Flow (GVF) solution in SewerCAD shouuld be adequate. I'd suggest using fairly small time steps.
However, for these situations where you can back up flow into tributary sewers and overflow upstream manholes, you are better off solving the full St. Venant equations as we do in our implicit or explicit solvers in SewerGEMS. That should give you a more accurate picture of sewer filling.
One good thing about the latest version of SewerGEMS is that a user an switch between the GVF solver and St. Venant solvers with just a few mouse clicks (if you have set up the model well).
How big is the model? What kind of pump curves are you using?
Best wishes,
Tom