190E-Engine problems.

I have the following problems regarding my 190E and I appreciate if you could share some opinion/advise.

1. The engine have no problem in starting and high RPM, but when idling, it is a bit rough at around 800 RPM, it seem like miss fire once a while. I have replaced the spark plug and vacuum hose under the air filter.
but no luck. Initialy, I though it was due to air cond engage but this happen even the air cond is off. Any advise what else to look for ?

2. How do I check/ensure the engine mounting is bad? I was told there are 2 types of mounting (round and square). Is 190E using
the round type ?

3. The engine oil level drop 1/4 after about
3000KM travel, and I hardly notice any white smoke from the exhaust. Confirm no leakage.
Actually, the car was left for about a year without start. Could it be the ring is jam ? Should I add some oil treatment to improve
it ?

Thanks in advance.

First I will check the spark plug wire set ?
motor mount on 190 only have one type is round hyd mound......How u check put a 13mm bolt on side of motor mount if u can put the bolt under the mount that mount is good if not replace it or check is any hyd fluid leaking from the mounts if leaks replace it.