Disclaimer: I have never seen any digital mixer other then on pictures...
The first part of my reply can be too general or simplistic for you, but you will understand why I decide to reply
Digital Mixer == Audio Interface which is (a) able to work standalone (b) can mix signals in standalone mode (many interfaces can work in standalone, but only as pre-amps, so One input -> One output). So you have already one Digital Mixer, your Motu MK3! Less inputs/outputs, less effects, but the principle is absolutely the same.
They call these devices "Digital Mixers" and your MK3 just an audio interface because DM also have some direct control possibility, just like analog mixers. Either "virtual" on iPad (new trend, f.e. Bahringer X32 Rack) or real hardware (like this Yamaha). And so it is easy to control live mixing without computer (something not manageable with your Motu).
As an audio interfaces, DP are working live only, unlike "compact studios" (f.e. Tascam DP) they are not able to record audio (so no "mix within").
But there is a "second mode" for hardware controls (pots, faders, buttons, etc.) of DPs which have them (this Yamaha) in which these controls no longer operate with DP parameters, but transfer signals to change parameters in the DAW. For Yamaha it is called "Remote control (layer)" in the reference documentation. And here is the point: this Yamaha can only send signals suitable for ProTools and other DAWs which support such commands (so called HUI protocol). Sonar does NOT have native support for HUI protocol and so you will be unable to use that... Till you use my plug-in:
http://www.azslow.com/index.php/topic,223.0.html (look later in this thread for currently implemented functionality, but as you can read the work is still in slow progress).