Since you bought the machine at this shop AND they are your closest "authorized dealer/repair shop", I would contact Janome directly. They probably don't want this reputation getting around & may be able to help you further with this experience. If all you took it in for was to "fix the needle threader" & then they state they didn't fix that...I would make sure you have/get a copy of the work order before you say anything else to this shop. You mention you also had it "serviced"...ask them to put in writing what all they do for this "service fee". Each time you go in now, ask for a paper in writing regarding "specifically" what they will be doing to your machine. This would help assist you with Janome at pinpointing why these people may not deserve to be an authorized dealer/repair shop. How far do you have to go to the "next" authorized dealer?
This really makes you want to go "sit in their shop & visit with their customers" if they don't make this right! They should be apologizing over & over & giving you a refund to boot at this point; as well as fixing what you originally brought it in for.
That is what a good reputable business would do to keep you as a customer!