Sounds like a blown HG to me. Usually on D24s and VW diesels the headgasket seems to blow between an oil passage and a coolant passage, but I think that's when they blow because they were overheated. In your case it was probably the boost that popped it, so it makes sense it would pressurize the cooling system with combustion gases instead. Either way, I'm impressed it took that motor as long as it did to finally lay down, considering the kind of use that it was being subjected to...
Hey, at least now you've got a good core motor for when you wear the next one out.
Would be worth trying a cap just to see what it does. A new 150kpa cap is a good move, it's what the car was designed for and won't overpressurize. Maybe that's all that is wrong, or would at least help it limp along for a little while longer. Wouldn't want to let it get an airlock in the cooling system and then overheat and warp the head though. I'd be watching the temp gauge close while driving if there's any question of an HG issue.
I think I remember you saying you didn't put in a new pilot bearing that fit the input shaft on the T5 tighter when you swapped transmissions? Wonder if the trans bearings would have held up better if there had been more support on the input shaft. On the new motor it might be worth finding a bearing that is the right size and high quality, and see if that helps it out.
Looking forward to seeing how it all turns out...