Have you taken photos of the condition of the machine you did get back that was not cleaned. If possible use a dated camera or have a digital calendar clock in the photo also. Be sure to get good shots of the inside of the machine to show the dust and lint and lack of oil. A good dated photo can speak volumes. Document now, if you haven't, all contacts you had with these people and every word that was said. After a few weeks, those things are hard to remember. Accuracy counts.
No business should ever act like they did, but especially not regarding sewing machines!
Good luck.