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Far out, what a hassle it's been.

Yep, it's easier than you think to under or over torque.
I think my 11.5" was around 45lb to get it lined up.
My .22lr upper, obviously no gas tube to worry about. Around 35lb.
My 16" Walther...

Yeah, that wouldn't help!
Sounds like it was backed off too much to line up the gas tube.
Does your handguard use shims for the barrel nut?

I'd at least expect Ken to check the upper and barrel are in alignment and change out the barrel and bolt carrier group to see if it improves things.

This doesn't sound good.
Have you contacted NZAR15.com? ie Ken?

+1 for the 65gn Gamekings.

Go well in my:
1 in 8" Sako 75
1 in 9" 16" Schmeisser (Lothar Walther barrel)
1 in 7" 11.5" Frankenstein, think it was a Colt 2nds barrel from Spanners.