Are they being sold as akoyas? Akoyas are not usually that dark, even the blue gray ones.
Can you post photos of the entire strand?
What is the metal of the clasp?
Size of pearls?
I'm thinking maybe either dyed fwp or Tahitians.
FW keshi are usually not so white-white, but I see what you mean about size. They are beautiful regardless.
The larger ones that cost less-- were they less lustrous? Less well matched? Shapes less pleasing? There had to be some reason why the larger pearls were priced lower.
The blemishes of fake pearls and real pearls are often different. Can you post closeups of some of the blemishes? Especially the more prominent ones. Check near the drill holes in particular.
The dress is lovely! I don't know what length I'd go with. A choker could look stunning.
But what if she wants sparkly bling instead (real or imitation)?
Have you considered the idea that her fiancé may have bought her jewelry to wear? A friend of mine received a spectacular akoya necklace from her fiancé for their wedding.