So I thought "gee, lets see how this looks all together before I start on the gas block etc"

Barrel nut is on finger tight, and hand guards are "stuck" on with a friction fit.

Suggestions on best way to lock this thing down and pull them hand guards back off...

I'm thinking clamp receiver part to the bench like I would've had I used a wrench to put the barrel nut on for reals and then start pulling.

What handguard?

Something like a BCM MCMR, I'd hit it with a heat gun or torch and gently pull it off the barrel nut.  I'd wear gloves after heating the rail by the way.

If the handguard is aluminum and the barrel nut is steel, a little heat should loosen up the fit a bit.  It might also help to put the whole assembly in the freezer for a while before heating it up.

Did you try holding the rifle vertical and “mortar” the rifle onto a rubber mat while holding onto the hand guard? Like a slide hammer.
Just be careful with the crown.