Agreed this would be nice. I deferred it when working on that task largely because in all cases there is a workaround (the one Bugreporter just described) and thinking through all the edge cases is a pain - you thought of some, but another thing you would need to do is emit a PageLatestRevisionChangedEvent on the page whose current revision was changed, and if you're creating a new page go through a lot -- but importantly not all -- of the steps involved in creating a page. So this breaks a lot of subtle, potentially unstated assumptions in MediaWiki core code and anyone trying to do it (which won't be me) should beware.

if you're creating a new page go through a lot -- but importantly not all -- of the steps involved in creating a page.

This would be comparible with undeleting a page (which supports undeleting some of revision). Indeed, the way to split page history before MergeHistory supports it is first delete it and then undelete some of revisions.