At the moment the only sensible option for storing the release dates of identical releases in different places seems to me to merge the releases (which have been created from the release events during the NGS migration) into an arbitrary one and put the dates for the other places into the annotation of that.
Everything else just leads to unnecessary duplication of edits, inconsistencies etc.
Editing release relationships is already bad enough, what with the explosion of the number of releases.
Of course there are cases where you can't be sure the US release is really the same as the European one. But in just as many cases you do know.
Ideally there would be multiple (place, date) pairs per release, just like there are multiple (label, catalogue number) pairs.