From your summary I understand that your customer's CTO plays a tricky game.  He will continue to delay answer, answer as if he hadn't understood the question, or worse.  Then he will go to his management and say:  "look at those guys that you've selected !  After 5 month they didn't produce anything tangible and they do not even understand our requirements".  Then the agreement will be cancelled.  Your company will be sued and maight be obliged to refund the amounts already invoiced.  And your customer will restart a project with your competition.

I had a similar situation in the past.  It's not easy, but not desperate either. 

So the first thing you should do is to notify the customer of his lack of cooperation.  You have to be very formal, with an official letter by some senior manager, and with detailed annexes showing the exact list of task they didn't perform (or perform late), and the copy of the emails of every unanswered questions.   Normally this will lead to a crisis meeting involving not only IT, but also the people who favoured your solution (I suppose there must be some). 

Two possible outcome:

1) customer IT department is put on track by their general management.  But be carefull.  Keep a file with all the delays or problems created by the bad guys.  Just in case..., so that you can react more easily if necessary again. 

2) no crisis meeting.  Or IT department still do not cooperate.  Then you have to see with your legal team and your boss what your ultimate weapon could be.  What I did:  I informed the customer that they didn't fulfill their part of the agreement, and that I would withdraw my team in order not to waste my company resources on unproductive assignments.   I was really ready to do it and got the agreement of my general manager.  But the customer's management understod that if a supplier renounced to a lucrative job, then there should be a real trouble.  And the customer's team then finally did cooperate, in order for them not to be held as responsible for the mess...

Good luck !

From your summary I understand that your customer's CTO plays a tricky game.  He will continue to delay answer, answer as if he hadn't understood the question, or worse.  Then he will go to his management and say:  "look at those guys that you've selected !  After 5 month they didn't produce anything tangible and they do not even understand our requirements".  Then the agreement will be cancelled.  Your company will be sued and maight be obliged to refund the amounts already invoiced.  And your customer will restart a project with your competition.

I had a similar situation in the past.  It's not easy, but not desperate either. 

So the first thing you should do is to notify the customer of his lack of cooperation.  You have to be very formal, with an official letter by some senior manager, and with detailed annexes showing the exact list of task they didn't perform (or perform late), and the copy of the emails of every unanswered questions.   Normally this will lead to a crisis meeting involving not only IT, but also the people who favoured your solution (I suppose there must be some). 

Two possible outcome:

1) customer IT department is put on track by their general management.  But be carefull.  Keep a file with all the delays or problems created by the bad guys.  Just in case..., so that you can react more easily if necessary again. 

2) no crisis meeting.  Or IT department still do not cooperate.  Then you have to see with your legal team and your boss what your ultimate weapon could be.  What I did:  I informed the customer that they didn't fulfill their part of the agreement, and that I would withdraw my team in order not to waste my company resources on unproductive assignments.   I was really ready to do it and got the agreement of my general manager.  But the customer's management understod that if a supplier renounced to a lucrative job, then there should be a real trouble.  And the customer's team then finally did cooperate, in order for them not to be held as responsible for the mess...

Good luck !