sounds to me like you will have it all worked out.
If you don't have the factory alarm box then it would make sense to me that the wires are not being connected.
The factory alarm should make the proper connections when the alarm is disabled or should break the starter connections when the alarm is enabled.
Though I have seen benzes driving around with the factory alarm going off, so I am not sure if the starter kill is part of the factory alarm on some models.
In anycase, I like the idea of the push button for starting.
Though you will have to tell the valet parking guys about the pushbutton starter.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.