I'm sorry to disagree with Agupta, but there is really nothing to go wrong with the chain tensioner itself. You just need to remove it and push the plunger all the way through then reinsert it from the back side. It ratchets to take up the slack as the chain wears. By pushing it through, then reinserting it you allow slack that will be automatically taken up by the tensioner. If you don't do this you will probably break something.
Agupta's recommendation for replacing the rails could have some merit, however, unless there is one broken, which could be the cause of the problem in the first place, the tensioner will take up any slack that may be added by worn chain guides.
As for the upper timing cover leak, there is no reason for it to leak except if it were installed by someone who did not know how to put it on correctly. The main thing to know, which will get the upper cover on correctly, is using the correct sealant. MB sells a translucent blue sealer for this application. It is not a silicone, it is translucent and has an acrylic smell. This sealer will allow you to push the cover rearward into place without disturbing the U seal at the bottom. Use the translucent blue on the U seal and cover to head mating surface and gob it in the lower rear corners. Use your fingernail on the shaft seal that you have oiled to start the shaft seal without it "folding". If you use this procedure and this sealer, you can put it on every time with no leak as a result.
I'm sorry if I gave you the impression that you will have no bent valves. I really don't know if these are interference engines, and I'm sorry that your engine is going to be the one from which we learn this. Best of luck with that. If the valves ARE bent, it would be worth pulling the head and replacing valves as needed, and all valve guides. At this mileage, this engine would most likely respond very well to a fresh head. The cylinder head on these engines come off and go back on quite well. How well these engines run is all in the cylinder head. You will most likely find very little cylinder wear after removing the head. I'm just throwing in these comments, I'm sure your hope is a fresh chain and back on the road again.
How low does your oil pressure go when hot? MB says that as low as .7 BAR is okay hot. That is about 10PSI. If the oil pressure is not falling below that, I would not recommend heavier oil. The synthetic in a reasonable weight should be fine. Synthetic oil is really good stuff, but extended change intervals with any oil allows build up of microscopic particulate that is not caught by the oil filter. I would be interested to know what you find when you closely examine the chain. I would be interested in whether it failed because of metal fatigue or wear through.
BTW Agupta, this is neither a 16V or an 8V, it is a 12V. The 2.6 in the 190 is an M103 six cylinder engine, a smaller displacement version of the 300E engine.
Best of luck and keep us updated,