now I have more questions then answers. I've never actually thought about smart contracts if the parties are on different branches.

My targetted attack is those with self custody. Standard divide and conquer strategy, and my division is at it's weakest and most vulnerable point.

Two different versions of events regarding the contract could happen between the branches. On the new, fiatconsensus branch, the contract conditions could be considered to be satisfied, while in the standard bitcoin consensus, the conditions are not met. A sovereign victim of this attack could simply sell the returned collateral (if the fiatconsensus allows it: they'd probably come with strings and monopoly privilege to certain corporations attached), and let the collateral from the real bitcoin fall back to them as a result of violation of the contract. Their valuing of the real bitcoin they hold and discarding or refusing to operate within the other chain, voices their say against the fiatization of money. This is assuming you have a rock solid smart contract set up. In the case of rugpull via just convincing other people to join your fiatconsensus chain and driving up demand for it artificially, you need only outhodl the attacking corporation and all of its useful idiots, even if they overpower you in control of the security budget. You need only outhodle them because they will destroy themselves in time. High time preference, reality-disrespecting crap like that always does.