I haven't followed the details of the problem but from a quick read, I
surmise that

1. PO did some work on engine, possibly removing head.
2. No initial leaks
3. Leaks from the interface of block to head of coolant.

Based on these marginal suppositions, I would just suggest a possible lower
cost solution. A common practice of many non-VW shops is to reuse the head
bolts and retorque them. Another practice is to bolt the head down hard
before beginning the torquing sequence. In my experience, all diesel heads
are warped (usually center high). The head bolts are actually torqued right
up to the yield and stretch point. Reusing the old head bolts now puts them
well into the yield part of the curve and they relax or release with
subsequent heating. (Yes, I know many backyard mechanics reuse the head bolts
and never have a problem, but I have been assured that pigs fly also). When
replacing depressingly bowed heads in the past, I carefully followed the
torquing instructions with new head bolts. Fired the engine up to operating
temperature then retorqued (often added half a turn) then drove the car for
about 30 minutes, retorqued and finally a third time. With one VW rabbit
block I retorqued 5 times before I had no more bolt rotation. I recall I
asked an old diesel mechanic (while working on my Peugeot diesel) how many
times should I repeat the retorquing cycle ... he replied with exasperation
... as long as it takes! Well, what if I don't? and he replied "head gasket
leak sooner than later".

Anyway, the head is astonishingly flexible. The proper torquing spreads it so
gasket compliance takes over.

Just a thought, value limited to transference costs.

Frank Grunthaner