Fault finding obscures the real problems

Asking effective questions is often helpful, but today I had an experience where this approach wasn't getting me anywhere. In fact, the problem was that I was asking questions at all.
In the old way of doing things the software may have been late or poor quality or lacking in features but the customer could be clear that this was the "development team's fault" (in some vague, arm's length kind of way). But now the development team are starting to work more closely with the customer this excuse for the product's problems disappears. In fact the original finger of blame might conveniently have been hiding a very different problem indeed.
The development team have asked for a single voice to provide priorities and direction, and the customer who could provide that voice is finding that they represent a diverse user base whose different priorities are intrinsically hard to balance. No wonder this question is so difficult to answer:
Can you help me to understand if meeting the original deadline is less important than having this additional functionality or are there some features we can defer in order to meet the deadline?