Well, here is the result of my discussion with the MB dealer.
Yes, they are responsible for that front pump seal leak and they will be paying for that.
Now they took a very close look at the transmission and besides the leak next to the modulator they say there is another leak at the filler and the kick-down solenoid also "looks wet".
So they now quote $ 700 for the B2 piston replacement and all the leaking seals, or $ 900 for above plus all other seals replaced that don't leak now but might be leaking once the others are replaced.
There argument is that you fix one and now the next weakest will leak etc. unless you replace all at once.
The $ 900 still leaves one seal out as you need to replace that from the inside of the transmission. Warranty is 1 year.

A factory rebuild transmission is $ 1,930 incl. tax. Warranty is 2 years.

They would pay for the R&R of the old/rebuild transmission as they have to do that anyway to get to the pump seal they are responsible for.

So is that my best longterm option to get a rebuild now although this transmission has only 47,500 miles on it?

The torque converter would still be the old one. ($ 1,450 plus tax for a new one)

What do you think?
Thanks for all your help.
Reinhard Kreutzer