Mark,
Oil pressure is much greater than coolant pressure! 3 bar on your oil pressure gauge is 44.1 psi and it goes higher than that (bypass valve to limit it in the 60's, I think). Coolant pressure tank cap will be rated 8-20 psi on most cars, hence oil into coolant is more common than other way around. Your other question- there is no connection with the water jacket at the gasket of the oil filter housing. In the "old days" it was common for us to retorque the heads on small block chevys we were running on street rods and "kit cars" after running them a few thousand miles. Don't know on diesels (only have had the 300SD) but maybe someone here has tried that with success, but that sounds like a "minorly leaking" head gasket as opposed to a "blown" one and maybe a "cinch down" will work. (wishful thinking)

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)