Having struggled with trying to get MC to do whole-home media distribution properly for years, I think you're on the right track with using a matrix switch to distribute the audio. I'm doing a similar thing.
1. I think you'll need four separate audio devices (PCI soundcards, USB soundcards, DACs, DLNA streamers etc). A single 8-channel device won't give you independant audio in each stereo pair, you'd only be able to stream the same audio to each zone - so in that case why bother with a matrix!
2. Enable Media Network on the main PC. Load its library on the office PC (no need to enable Media Network there), so you can browsed the media from the central library but play locally of the office PC.
3. I assume your wife will use the central media too rather than having media locally on the Mac? In which case I think the way I see this working is do the same thing as 2 above but also check the option "Show zones from the server on the client" in Media Network on the Mac. This will allow the Mac to browse the media on the central library but also output to any of the zones on the central PC - so feeding into your whole-home amp as normal. If, however, your wife has a local isntall of MC and media on the Mac, then instead enable DLNA Renderer on the main PC and enable Media Network and DLNA controller on the Mac. This will allow the Mac to browse local media but send output via DLNA to the zones on the central PC (each zone will appear on the Mac as a DLNA destination). however, a small disclaimer is that the Mac version of MC is a port and is work in progress, and I don't know how much of this functionality is there.
Out of interest, how are you controlling the matrix switch/amp, i.e. controlling which rooms get which audio, grouping rooms etc? Does it have a mobile app, or some other remote controller?