Apparently you too believe the leak's source is the crankshaft's rear
seal, not the cylinder head. In view of the engine's smooth idle, and
you should confirm this, no oil in the coolant and no coolant in the
engine's oil and no overheating the rear seal does appear to be the most
likely culprit. Based on the leak you must have to refill the lube oil,
Again I suggest a second opinion but NOT by a transmission shop; take
the car to an independent M-B shop and simply ask them to identify the
leak and quote the repair cost.
Transmission shops have one expertise - transmissions. IMHO they have
neither the knowledge to repair the rear seal nor the inclination to do
anything other than remove and replace transmissions, regardless of
what's really needed.