How do we keep a collaborative draft from splitting into several final versions?
Pick one person to own the final pass. When the team knows who integrates feedback, more people can contribute usefully. Without that person, quick revisions can leave you with several believable finals that carry different facts and different approvals.
Give contributors a clear way to send comments and to say which version they're looking at. Skip asking several people to rewrite the same passage on their own, unless comparing is the point of the exercise.
The editor reconciles the changes against the brief and the verified sources. Anything two people disagree on gets flagged for a decision, and the version that arrived last doesn't win by default.
Approval belongs to a specific version. If a consequential claim changes afterward, the revised claim goes back to the appropriate reviewer.
Archive superseded drafts or label them, whichever your team's process calls for. The recipient should get one identifiable version, with its status and the remaining decisions visible.