Thank you for all your help and comments.
On Sunday I had my outbound flight from FRA to BJS and I have to admit that I was a bit impressed about MS in C. When booking the flight I was a bit worried about that low price. Just a bit over 1000EUR.
As you mentioned C on the B738 from FRA is equipped with seats like an US domestic F. And the IFE was AVOD!
Arriving in CAI we were brought by bus to the immigration hall. The transit passengers were escorted to another bus bringing us to the international departure hall. After passing through security (it beeped but nobody stopped me) we were asked to go to a transit desk. There I received an invitation to the lounge. Next to this desk is a tour desk where you can book one of the quick tours. I will take one on the way back on my long layover.
The lounge was… let’s say basic.
Boarding again was by bus (I think there are no gangways at all).
The flight was on a B777. The pitch was approx. 45” and the seats were comfortable (they remember me a bit on old LH long haul C class seats). C Class from CAI to BKK was full. But there were only 4 pax in C on the extension to BJS. So I was able to use that secret switch in the armrest to lower the seat on approx 170 degree. Very comfortable!

I really can tell that I do not regret that I booked on MS. The price/comfort relation is absolutely OK. That compensates the longer route and CAI airport.

(As always when working in China) I look forward to my flight home…..