Yes, we have done leak down prior, no, its not the rings, cylinder walls are fine, crosshatch looks good.
After doing some more research and making a couple calls, Apparently this IS a known issue and , yes, MLS gaskets can leak between layers if the head lifts slightly .
Top of the gasket/head leaks into the lifter galley area around/through the head gasket at the top of the block .
Never ceases to amaze me the odd things i run into.
Now, that said- if the heads don't show to have lifted between 3 and 5 ,,,, im back to square one with this troubleshooting nightmare.
Rings appear to be fine from an upper inspection, walls all look good and no- the smoke is not steam- steam doesn't fill the breather filters full of oil and there is no indication of water vapor in them, the oil or oil in the water.
Near as i can figure- engine is turned off, back pressure stops, oil rolls back down the leak path and lands on hot piston or, something HOT and viola, the smoke comes back up and out the breather.

Best i can come up with- everything looks great, tests great except for hearing air come out the lifter galley with a pressure test and its not coming out both sides per say- its coming out right above 3 and 5. You can stick your finger in there pretty good and feel it better- stick a piece of paper in it and it will blow the paper around.
Going past the rings, it would be pretty well diffused to the whole internals i would imagine.