Do I need vents to devices in the primary waste line?
I'm answering my own question since I changed the design based those comments here and on some other research (code and recommendation).

Sink C now has it's own pathway to the "junction box". The vent on Sink C joins the main vent stack above all and acts as a wet vent to the junction box too.

Toilet B doesn't need a vent due to distance to the main stack.

Toilet C is of a little concern because it is "wet vented" by the main stack, but since the piping there is 4 in I doubt that it could create enough back pressure to break the Toilet C trap.