The rain stopped. After checking the valve cover gasket and making sure it fits properly in the valve cover grooves, I put in the retread thread repair combo mixture along with the resistance fluid on the two bolts. In addition, I put in the threadlocker on another two bolts located in the rear valve cover that were the source of the oil leak (I believed). Soon after that, I torqued all the 12 bolts on the valve cover at 10nm crisscrossed. Fired up the engine and run at idle for a few minutes to warm up at normal temperatures 85c. Small puffs of smoke appeared on the passenger side rear engine. No burning oil or smell so far. I then droved the Mercedes for a few miles, returned home and checked for burning, smoking oil and leaks. No leaks, no burning oil and no smoke. I then tightened again the 12 valve cover bolts at 10nm while the engine is still hot. Problem solved, FINALLY!!!

As for the high idling issue, I found out that I was missing the fuel/gas cap. I bought a new fuel/gas cap. When idling now at its normal 800 rpm, it sounded like a shaking/rattling and low power when accelerating, like it needs a tune up or something like that. But that is another story at another time. I am just happy to resolved that nagging, persistent burning and leaking oil along with the smoke that had been bothering me for quite some time. However, I suspected it is a temporary band aid fix and I know exactly what the issue is if it pops up again and what to do about it. Thanks to all members of this forum who posted their comments/feedback/advice along with those who taking the time reading this post/threads.