Long-term Salesforce consultant here. Can't speak to the specifics in this case, but mixups of this nature are sadly quite common in the Enterprise world. In the Salesforce ecosystem particularly the fast pace of growth in customers with complicated needs has not been met by either by internal professional services within Salesforce or by the larger partner community (it is difficult to re-train people in proprietary technologies and rapidly build up companies around them). This means that many partners are billing high rates for subpar services. In this case they appear to be under subcontract to Salesforce, which could mean that Salesforce is partially responsible (IANAL).

I don't think there is any silver bullet here. Large enterprise projects generally need a lot of hands in the implementation, particularly when there are tight schedules involved, and when there aren't enough quality resources available, people end up using resources that aren't able to deliver on what they promised in the timeline they promise.

I've seen this happen before (generally from a distance) and all I can say my condolences go out to all involved. As far as advice goes to developers, consulting in general is filled non particularly interesting repetitive tasks that rarely attracts great talent, and the most talented people tend to up into management (i.e. architects) rather than hanging around in the trenches. As far as advice to people involved on the purchasing side, make sure you vet your consultants ahead of time. Usually if take someone who promises something too good to be true, it will turn out just that way.