Will a factory reset cause the registry settings to return to normal or will is still say it's a 930? Also, would this also allow a firmware update if the 930 gets one and the Icon doesn't. Would such a firmware update basically convert the phone to a 930?
Factory reset should put it back as a 929. You will stay on current build but will not get new builds until you change the registry again.
As for firmware, that's a "maybe". Actually, I think firmware is determined by "PhoneManufacturerModelName", since that is specific to the carrier. I didn't change this on my phone and still got the redstone builds. Actually I'd suggest putting that back to default values just so that you don't accidentally get the firmware update.
A few things might happen if you get the 930 firmware on the Icon:
1) Bricks the phone
2) Phone boots but no longer works on Verizon bands (or might not work on any bands)
3) Everything works fine and phone is now a 930 that works on Verizon (least likely I think)
I wouldn't risk it until someone else is brave enough to try.