The imperial family had the dress made for FL, and in case of an emergency, a replica dress was made, which was in possession of the empress. So they deliberately tried to humiliate FL by giving Helena the replica dress to upstage the fl. The fact that a mere maid accompanied the crown prince, the fl's fiancé, to her own coming of age ceremony, was also meant to humiliate her even more
The empress is in charge of providing the dress and keeping a spare in case something happens like a drink spills or something. Then Eris can change into the spare. The empress gave the spare one to Helena or the crown asked the empress. Either way, Helena wouldn’t get the dress unless the empress approved
I think the empress assumed that the crown prince and Helena would enter the ball first (with the FL kept 'waiting' for the crown prince to escort her), she would then enter the ball later after getting fed up waiting, and then be humiliated when it looks like she also copied 'the maid's' dress.
how the empress interfered? I really don't get that bit.