true, the "sibling chain" as you put it could allow re-orgs independent of bitcoin. but then I'm not sure what the benefit of putting all of the chain data inside bitcoin blocks would be, as opposed to just putting a hash so that nodes could simply follow a chain of headers and make sure that matches up with the chain other nodes are feeding them. can you think of what the benefits of putting all of the chain data inside bitcoin blocks would be in this case?

lightcoin

bitcoin

Building sovereign rollups on bitcoin

I mean Bitcoin itself is unaware of it. There'd be no way peg mechanism, so it's like a sibling chain. That's great, as people can just do it in a permissionless manner. 

I think the specifics of double spend protection or not would depend on the consensus system of the sibling chain. Sounds like a cool design space to explore. 