If it ain't broke, why spend money and time fixing it?
First, your car is 13 years old, regardless of mileage. Ignore the odometer, as sitting is WORSE for a car than driving it.
Next, your engine seems in good shape. The compression/leakdown tests came out within spec, so why are you fretting? Our 1990 190E 2.6 uses a litre of oil every 1500km's (about 900-1000miles) and it's running well. Until something happens, I'm leaving it well enough alone.
If your plugs are not fouling and the compression is good, you're probably best off just adding oil every few tankfuls...
1998 C230 "Black Betty"