Well it looks like the Arris DCX-3510M is capable of High Definition output over a HDMI connection, directly to a TV or via a Receiver, which implies that the output is HD and not encrypted. In the past anything that could output HD Digital video would only be allowed to output encrypted video. If it was encrypted it had to be SD or analogue. At least that is my understanding, as a copy protection mechanism. I have no idea what the implications are for Copy-Once Cable TV content when using this device.
It looks you should be able to feed that HD Digital video signal into a Colossus or Colossus 2 and have MC change channels using the built in IR Blaster on the Colossus, and record programs as scheduled, for later playback. Or have MC change channels and watch it live of course.
It seems though that the Arris DCX-3510M is a two tuner device, and also a DVR with built in hard drive, and the ability to add an extra eSATA hard drive. I don't think you could get MC to use the second tuner, but I guess the Arris DCX-3510M might be able to use it to record programs, while MC is using the first tuner.
You may be able to use the DVR capabilities of the Arris DCX-3510M to record programs, and if the internal and/or attached hard drives are accessible as network drives, then MC may be able to import and play programs directly from the Arris DCX-3510M. That could be neat. The Arris DCX-3510M does talk about whole home DVR capabilities, but using MoCA (Multimedia over Coax Alliance) network infrastructure. The process looks proprietary and controlled by your Cable provider. Requires authorisation.
But you aren't really going to get the advantages of a multi-tuner set top box. I suspect you might just have to use it like a single tuner box.
If you do consider a Colossus, don't just take my word that it will hang together and work though. Search the forums for discussion on the Colossus and Colossus 2, as I think some people have had issues using them.
Disclaimer: As I mentioned before I haven't done any of this. I'm just going by specifications and past second or many hand information. so I may be completely wrong. Up to you to check it all out. Particularly DRM issues. Shaw does transmit Copy Once channels.