I'm now holding zero better after a few upgrades. Not a scientific test, but added the MCarbo trigger and spring kit, added the hand guard stabilizer bracket, and reattached the MI mount with PA Microdot locked down tight with blue Loctite.
Any fiddling with the windage and elevation turrets giave a more or less nonlinear response but got to a decent zero after about 30 shots. By nonlinear I mean that just touching the red dot moves the output group. That tells me what I though to begin with, that the spring on the MI mount needs to be stronger, or maybe it's just this particular mount?
Primary Arms told me they have had several complaints about this mount, and that MI told them that it was almost always a mounting problem.
Will say that, after taking the hand guard off, there's no doubt the stabilizer brace is needed, and maybe more than one. There are only a couple of the bolt/screws that really hold the hand guard to rotating hinge. That latter is the receiver, regardless of what it looks like. The rest of them are holding the two halves together.
Shooting without a bench at 25 yards I was centered on zero with maybe 3 in groups. Without all the changes I was all over the paper, so at least this is better. Waiting for a new mounts to come in so I can swap and then test this again.