Just to let you know, the new Chrome version was installed via scheduled updates, so the repro case is gone. Chrome should switch to running the new version on the next restart. The help page now displays "Nearly up to date! Relaunch Chrome to finish updating."
To clarify, what happened before was that Chrome would very briefly display "Checking for updates", then display "Updating Chrome" and hang. This seems consistent with what you were saying, that Chrome first sends an advisory update check, then triggers the check-to-update right after. Except in this case, it wasn't triggering the check-to-update. It's unclear to me whether this was a Chrome bug or an updater bug.
As a separate issue, my updaters are stuck on build 5668 (current canary build is 5672), but you said there was a differential update bug, so I assume that's the reason for that.
John