A statistically significant number of 350SD/L engines succomb to connecting rods which bend causing the cylinder bores to wear in an oval pattern and other such unpleasantries. I'd look for a tech with specific experience with the 603.97x motor and lots of it. He'll be able to tell if your problem is cracks in the head or bad connecting rods. Hint, an MB dealership is probably not the best place to go looking for this tech. I think it's a worthwhile gamble to get a $200 opinion before throwing another $1800 at potentially uneccessary cylinder head work.
FWIW, when the head of my 87 300SDL had cracks, coolant consumption was not so bad, temp would go to 120 quickly and there was tremendous pressure in the cooling system. Idle was pretty bad but the car was driveable. No mixing of oil and coolant. It was easy to see the cracks in the combustion chambers.
Lots of info for you if you use the search feature. I'm sure someone in the Diesel Discussion forum can recommend a tech in your area.
BTW, removing and replacing the cylinder head is not a particularly difficult job.