You are dealing with a chain of suppliers.
They are probably basing their dates on the input they are getting from the suppliers of the chasis.
We deal with these types of things all the time a work. You have your typical lead times for parts and then things don't arrive on time or the supplier has a problem and didn't bother to infrom you in time.
The changing dates are driving me crazy and I have not even ordered mine yet, so I understand your frustration. It would be nice if they only charged your credit card once it has shipped.
I don't know what they could do differently except communicate better. It sounds like parts are not passing QA and that throws the dates odd as well.
That's my $.02. Hope this is not redundant.
Hope you get your m15x sooner rather than later!