So little fluid comes out of radiator drain!

[JHZR2]

Does the block have a coolant drain plug?

This. I’d suspect that the vehicle has a lot more in the block and turbo. Potentially much more still if it has a big heater core distanced away, and/or a rear heater.

 

[zzyzzx]

How about starting it up when cold and running it for a few minutes while closely watching the temp gauge ?

I would run it for a few seconds, not minutes.