Hi U-Man,
MCU and C4 protocols use MIDI to send and receive information. AFAIK there is no Mackie SDK, but rather a documentation set about which button / fader / encoder uses which MIDI CC. Faders use 10 bit for a higher resolution than the 127 values of the standard CC. Both protocols are different. The C4 had very little DAW support (only Cakewalk, Logic and Tracktion) and was discontinued years ago.
The displays are controlled sending binary information encapsulated within the MIDI messages, and it's different between the MCU and the C4, which is why you don't see anything when you set it as an MCU.
If you want to understand how they work under the hood in Cakewalk, you can check https://github.com/Cakewalk/Cakewalk-Control-Surface-SDK
As for using only a C4 without an MCU, yes, this is possible. You can control track levels, pans, plugins and most of the button functions the MCU has. The cakewalk help file is pretty comprehensive about using the C4 (attached).
Hope this helps!
MackieControl.chm