If I have understood you correctly, you currently have power working using setup code 0842 and entering the EFC 00051. So that is already done, and changing to a different setup code won't affect this.
Now, asssign 0135 to AUX. That should work immediately, but if it doesn't, assign the EFCs for Vol+/- and Mute. That should give you the volume group buttons. I think you'll find that some other functions work as well. However, with the Comcast remote, the Channel Lock feature is probably active, and so CH+/-, digit buttons, and OK/Select will be sent to the cable box, even though you're in AUX mode. If you need those buttons for the receiver, we can post the EFCs for those also.
Finally, just leave the setup code at 0135.