I don't have all your answers, but here's some info.
State law applies, if the state you are residing in doesn't allow certain firearms then having them federally registered makes no difference, you can't take them into that state.
SBR's are "class 3" weapons, they fall under the NFA like SBS, MG's, Silencers, etc. No difference in federal law, if you can legally own an SBR you're good for all the others as well, barring state laws against certain types. "Converted" also makes no difference on a Fed level, the conversion device would be what's registered for an MG. State laws that limit one to C&R MGs would probably have a problem with those though.
Inter-state movement of most NFA firearms requires an approved form from ATF before you can move it, either temporary or permanent. For example, going to Knob Creek from another state would require an approved form to be legal. If state law prohibits the particular firearm, ATF won't approve it.
As far as out of state corp registration goes, I would think ATF wouldn't have a problem with it. Never came up before that I recall.
Your Corp can buy guns, no federal registration other than NFA items though. State registration might apply though, and should treat a Corp as a natural person unless they specifically disallow it.