Pim,
I agree with you that for forward routing the values to use for the WS-A EPR of the destination are already known and that there's no need for a separate PMode parameter.
The intention of the current text however is to define the PMode feature, not how and where it is used. I'm also not sure if the PMode feature should be named RefParam, as it is more general about the use of WS-A. So WSA might be a better name.
As you indicated the EPR parameters are mostly needed for routing responses (reverse routing) and the RefParam feature will define the values to use for the EPR identifying the destination of responses which can be either WS-RM or ebMS signals. In the first case the PMode feature will define the values for the wsa:AcksTo and for the latter case the values for the wsa:ReplyTo.
Sander