That seems like the most likely scenario. You mentioned having the fluid tested. Have you done so yet?
I wish you the best, but even if they did mistakenly add the wrong fluid, if it wasn't actually itemized on your invoice, especially after so much time, I can't see you getting anywhere. That said, maybe its a problem they're aware of (new employee making the same mistake to other vehicles), and hearing your story wish to make it right. Stranger things have happened.
Alpine44... If it was LMH, would you expect the fluids to separate the way the Seb describes?
My first wife was 'tarded. She's a pilot now.