thermostat stuck? ~90*C but lower hose not hot
 

The car is 1992 300D.

I drove about 40 minutes. Ambient temperature gauge read 36*F, and the coolant temperature gauge read slightly below the middle of 80 and 100 (degrees C).

The coolant level was at the return hose. This means the level increased. I checked the level two months ago: at the middle when cold, 1~2 cm higher at operating temperature.

At the top of the coolant in the reservoir, there were foam.

The upper hose was hot (it felt like ~ 90*C), but the lower hose was not hot at all.

Answer
 

Replace the thermostat.
Do NOT drive it until you have.

I took out the coolant from my car and poured it into clean cup, and compared to the fresh MB antifreeze mixture.
The coolant from my car was clean, but it had slightly green tint. After beeing agitated, the foam on top of the coolant from my car lasted long like soap water.

I went to the MB dealer who replaced radiator, thermostant and coolant this summer. Nothing was done to the cooling system since the service at that dealer.

Surprisingly, they said it was OK! I still see the foam on top of the coolant in the coolant reservoir. It can bee seen from outside. It can be seen by opening the reservoir cap. They said there was small amount of foam, but the car drived normally. They did nothing, and charged half hour diagnostic fee.

Shall I go to that dealer again, or shall I just flush coolant at other place?

Ok.
 

You can depend upon one fact, with every car dealer service garage in the world:
If there was anything, service, parts or opinion they could sell you, they will try to sell it.

Since they have checked it, and found no issue, :) Relax, there is probably nothing wrong.:)

I suspect the thoroughness of the last flush. It probably had some green stuff in it. I would flush it. It might have had a chemical added for cleaning the radiator and that wasn't flushed out as well. I always run three or more flushes, and have had to do five (5) flushes after using a rad cleaner before I was satisfied the coolant was pure water before adding antifreeze. Dealers are not going to put that much time into the job, they have to make ready for another vehicle.
Find a good Indy or DIY, it won't affect the warranty if there is one.
And as for the difference in temp of the hoses, what you discovered is normal, the top hose carries water from the head so its always hotter.
Its hard to figure what you saw re: level. Water expands when hot, and some is lost if the reservoir is overfilled, etc.
So get a fix on the level and watch it to see if it ever changes AT THE SAME TEMP (ie. always check it cold for example) Resist the temptation to add water until it gets down to the "add water" mark on the reservoir (lower mark). If it drops that far in less than a year (depending on how much you drive) you have a leak and need to investigate.