If it helps any, I could access the Commerci Trade Union docks and do the party quest during the recent v.245 Magic Slate event. So this bug making the Union PQ inaccessible must have just been introduced in the v.246 New Age patch.
Tsk, seems like they patched the bug by removing the Dimensional Mirror entirely, just leaving that platform empty.
I wonder if the bug about Commerci not being flagged as a town interfered with the mirror's functionality.