I don't think it would mess with your warranty any more than say installing an aftermarket cell phone, or hardwiring a radar detector. I wouldn't worry about it.
I can't describe the signal any more than I have (it's an AC signal, varies with speed). Should work, can't say for sure. I'd assume the other type of signal (with the driveshaft or independent wheel speed signal) would also be an AC signal.
The signal, as I mentioned, comes from the right front. MB put this wire connection (for the radio and PSE) into the rear VSS connector, no better place for it I guess. The ESP control unit is processing all the wheel speed signals, but the diagram says the Right Front is the one being passed to the interior.
If you don't have the tools (special tool keys) to remove the radio head unit, maybe when you bring it in you can ask him if he'll just get the head unit part way pulled out so you can access the wiring.
Why would you need to know the signals corelation with speed? Doesn't the calibration mode take this into account?
In regards to getting wiring diagrams for your car, you mean PLURAL as in all of them, or just for the VSS or what?