Well I built it!

And in the meantime I've got my issue sorted but I'm little put off by Faxon's answer. They claim I need to pull a firing pin spring when using hard primer milspec ammo and replace it for commercial ammo. They said there's a risk of slam fire using commercial ammo without the spring.

None of this is in the manual or advised when I bought the upper.

That's easy enough to do, but the idea of having a gun that could slamfire with the right ammo/missing that little spring worries me.