Ok for one, yes you can disable the 4MATIC system by turning the lever (in engine compartment) to the forward "TEST" position.
And according to Bentley's Mercedes Bible it is perfectly safe to run it this way. And the SLS will still work since the hydraulic distributor runs independent lines (where the "TEST" lever is) to the TC and SLS.
IF you want to convert to RWD tranny would be the way to go to eliminate the (TC) transfer case.
Or you can remove the front driveshaft between the TC and front dif and the front axles. You would have to keep the outer CV joint piece bolted to the wheel hub so your wheel wont fall off.
Or you could transfer a RWD front shocks, springs, knuckles, brakes, and sway bar. Also maybe the tie rods, center link, and steering damper.
But you'll still have a little added weight of the front dif and pan that is incorporated with the 4MATIC engine oil pan.
Does that clear things up?
But seriously, all in all it's probably cheaper to shop around and buy good use (or new if affordable) parts to keep the 4MATIC system operational.