IRT valve gap being zero (valves touching); Yes, it is a very bad thing particularly if you have a diesel engine. If your valve adjusters are too tight, you are leaving your valves partially open at all times. This will result in burned valves due to hot gasses passing around the valve head and seat, especially the exhaust valves. You will also experience lost compression resulting in a power loss. Readjust your valves to the clearance specification for your engine. A quiet Diesel engine is one that needs a valve adjustment.
Assuming your car is a W123 variant with the OM617 engine, the timing chain tensioner is hydraulic and allows no manual adjustment. It serves as the right (passenger side) chain guide and is accessible by removing the right side cam (valve) cover.
True Dat as Klein says above. In any case, no epoxy will withstand the heat cycling that a plastic radiator sees. Replacement is the only safe option. They are not very expensive. Nissens, a Danish company makes radiators for our cars. I have one on my 300CD and believe it to be better quality than the OE.
There is some missing information. You don't say what year your car is or if it is a turbodiesel. If it is a California car there are further differences in the air filter arrangement. Assuming the most common configuration, remove your air filter housing and you will find a short steel tube extending from the bottom of the housing. That tube should have an o-ring on it. If it has none or the o-ring is old and hardened, replace it. That tube then fits into another steel tube extending up from the engine block to provide a drain-back path from the air filter housing to the crankcase for excess oil. It is easy to miss the mating of those two tubes when replacing the lower half of your air filter housing. It is a blind fit you will have to feel with your fingers while seating the housing.
I would bet that changing fuel filters and running REAL diesel fuel will eliminate this problem. Our engines after all are DIESEL engines. Anything else is suboptimal regardless of it's "benefits" to the environment.