There's no antitrust front on the MV2 to MV3 transition. You're imagining something that doesn't exist.
> Given the ways in which the story has changed about this rollout, my default is to assume that nothing that is projected is set in stone.
That's fine, and Google itself said in the announcement that they're doing a slow rollout in order to collect data and see the effects, but it has nothing to do with antitrust. The MV2 deprecation was delayed because Chrome extension developers complained that MV3 still had serious shortcomings that prevented them from migrating their extensions from MV2, so Google paused to address many of those issues.