Update 4: (with pictures)
I got the NGK plugs (see attachments) removed to new genuine MB plugs and also got the bad 10W-40 oil and cheap oil filter replaced with MB spec oil and filter.
As you can see in the pictures, the tip/head of the plugs have some accumulation of black crud. That's why the mechanic at MB specialist said to change them asap since they were not the correct plugs for the engine and not in good conditions.
Outcome: engine is a lot more quieter, smoother and faster and the fluctuation in the rpm needle at idle has decreased significantly.
But the shaking issue at idle is still present... Straight after the service, the shaking at idle had reduced but two days after service, the shaking increased back to normal again.
The mechanic has suggested to observe the shaking for atleast 100 miles due to new plugs and oil etc...
Since the STAR check is clear, a faulty MAF, coil pack or plug leads is unlikely to be the problem, as suggested by the mechanic.
I'm now thinking that the below maybe the causes?
Fuel filter? It has been changed in the previous service but I have a feeling that a non-genuine filter has been used, since a non-genuine oil filter was used.
Battery? The car starts always on first click but I think it cranks for a little longer than a usual car...could a low battery effect spark plug/coil pack performance?
Crankshaft pulley and alternator belt? These two items were replaced in previous service (not the service just done.now) and I fear that either non genuine parts have been used or they have not been installed properly, because there is a chain rattle type noise from the.engine which is loud on cold startup and quieter when warm and always occurs when accelerating. But could this cause the shaking.issue by any chance.
Thanks