For you, using a Myro:Air wouldn't be for the high-end DAC but the ability to obtain metadata and transport control of the AirPlay stream. You cannot do that with an AirPort Express, the only other system that comes close is the DMS3.1 by Russound (which is another option for you).
The challenge with me helping you is that your use case is very complex -- almost too complex. That's why I was trying to give you insight (different perspective) of how I'm using my system personally with my family.
I still believe AirPlay (or UPnP) with metadata and transport control would suit you best and provide you with the best options -- with the added bonus of an amazing DAC.
Again, you would be streaming via your iOS (or Android) device using AirPlay since everything you mentioned is available to an iOS device.
- Full access to iTunes library and playlists
Available via iOS device (iCloud and locally)
- Support for multiple iTunes libraries, including playlists. My kids will soon have their own iTunes libraries and playlists (currently only on iDevices, later on Macs) and will want access those as well
Available via iOS device (iCloud and locally)
- Use of at least one Sonos Connect as a source to the two C5s
What do you need the Sonos for? What services are you using that are exclusive to the Sonos?
- Ability to airplay from any iDevice, requiring a source (e.g. Airport Express, RasPi with XBMC, myro:air) that understands AirPlay
All you need is AirPlay (and/or UPnP) streaming
- Possibly multiple Sonos Connect and AirPlay sources to allow multiple sources
Why do you need a Sonos again?
- One user interface on my iDevices that does all of the above, selecting sources and mapping them to zones. That’s either the Russound or, preferably, the Sonos app. I don’t want to select the source with one app and the zone with another one. Currently, I need to use one app (iTunes) to select the AirPlay device (Raspberry Pi with XBMC), then switch to the Russound app to select it as input to one of the zones.
I would use the Russound App to switch sources -- but mostly just to turn on a zone (which would be set to the source that provides AirPlay). Then you should just start streaming... so it depends on how many streams you want.
Perhaps I'm missing something -- please provide more details why you use the Sonos and why you couldn't AirPlay that content instead.