For the Xfinity remote assign setup code 11317 to TV. That should control Vol+/-, unless the remote has been setup with volume punchthrough to the AUX device. See the
9XX instructions for VPT (993).
The Power button will likely only turn off the TV. However, you can redefine the function of the Power button (not the All On button) by using the 994 command and assigning EFC 34629 to it. This will make it a power toggle button.
1) Press TV
2) Press Setup until 2 blinks
3) Enter 994 (2 blinks)
4) Tap Setup (don't hold it)
5) Enter 34629
6) Press Power
The remote should blink twice.