Holly crap batman,I'm glad to hear all's well that ends well.
reading the whole ordeal, was wondering what it all did longevity wise to engine.
Your still going so that's great news.
threads old, but trying to get head around what actually caused the problem.

These car are made in Japan Correct!.
Run and tested and good to go before leavening Japan correct!.

Before leaving Japan do they drain all fluids to lighten the load for shipping??.
Would give a good amount of time for crap to develop a peel off within the block ready to block the radiator when it's next filled up with fresh fluids.
Suggest the system was never flushed when it landed in Aus.
And your system partially blocked from day 1 here.

And or when turbo was done cooling system dried out and the crap peeled off then and problem started OR multiplied then.
Crap going to the radiator to block it up.

Humm
Recently removed TB42 from Mav and replaced cracked head.
with in a week that much crap had dried and peeled off the bottom of cooling area in block, that I rolled engine on stand and blew compressed air for the next 10 min's to remove as much crap as I could before reassembly .
Was very amazed how much rubbish feel on the floor, given had looked after the engine maintance wise flushing etc.
Anyway Cheers.