NAPA sells a compression tester adapter for around $20. Dont quote me. I am pretty sure this isnt your last Benz, so consider this an investment. A cheapo compression tester for gas engines will work, but you need to swap out the gauge to something with at least 500 psi capacity, 750 would be better. You will also need to change the schrader valve in the tester for one for semi's, a high pressure one. NAPA will have the gauge and valve you need, you can buy the cheapo tester at Orielly. Make sure the cylinder is clear before the test by cranking the engine a few revs with all the injectors removed. 5 thumps of the gauge should be more than enough to tell you what its producing. My guess is its well under 300 psi.

MMO works pretty good on light carbon deposits, but beyond that nothing short of a tear down is going to loosen rings once their stuck real good, if that is in fact your problem. If ether killed it, MMO wont fix that either.