I can't believe your mechanic didn't look for where the leak was coming from.
The most common areas are the front timing chain cover where it meets the head, and the head gasket at the right rear corner (passenger side).
I don't ever remember reading on this site (or any other) about rampant oil leaks at the front crankshaft seal.
Maybe your car was leaking at both of the above places, as well as the head gasket, and your mechanic didn't notice the head gasket leak.
In any event, I'd have it checked again, and if it is the head gasket, I'd get a refund for the cost of the front timing chain cover as it would have been done anyway for the head gasket. There is no reason to essentially pay for it twice.
I don't see any way that you could persuad the dealer to pay for half of the cost of the repair, considering the year of the car.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".