I'd like to remove the rear sprocket carrier hub (one of these https://www.ebay.com/p/2215413698?iid=182740840014) and powder coat it. Any caveats? Assuming the sprocket wont just fall off cause the chain holds it in place? Any issues with powder coating it?

I did it to a stock carrier. Just make sure you don’t coat any mating surfaces. I did add a quick change but that’s behind the carrier cover.

Well only one of the 5 nuts actually came off for the for the rest the whole bolt appears spins with the nut after I cracked it. Not sure what's going on here? Is it not possible to just remove the outer flange without taking the whole sprocket off? Now I'm worried if I try and re-tighten the bolts that they wont actually be tightening to spec unless I take the whole sprocket off and tighten those nuts. Help

Tried tightening the nuts and two will tighten the rest just spin. From what I'm gathering you need to take the whole sprocket off and there's an allen head for the bolts that attach it to the sprocket. Ugh.. If I had known I had to take the whole sprocket off I wouldnt even have even gone down this rabbit hole. Just wanted to remove and powder coat the outer flange. 
Now I'm afraid I'm going to have to take the whole thing off to get it tightend which is a much bigger pain.

Yes, it usually needs to come off (I haven't heard about access from the other side), but it's just the one big axle nut.