That's the question, though. The NSA is known to have strongly conflicting objectives. On the one hand, they're supposed to secure US government devices and sometimes assist US companies in securing devices. On the other hand, they're supposed to surveil foreign citizens using such devices and the devices of US citizens who communicate with foreign citizens, as well as assisting other US agencies in doing that.
In a nutshell, whether they will increase or subvert your security depends on factors outside of your control. But unless they have found ways of surveilling foreigners without compromising the security of any Apple device, it's almost certain that they won't disclose their own 0-day exploits to Apple.
The US government have already "assisted" plenty. Every assist is a setback. IE. Snowden's revelations, encryption standard weaknesses, backdoored devices, etc.
No, but that is exactly why Apple might be a little bit reluctant to go for assistance. It's a bit like going to the neighborhood burglar to ask him how to secure your house. "I'll be right over to take a good look at your property" is not the answer you think it is.
That's an absurdly narrow view of the USG security positioning. USG pours billions of dollars into defensive infrastructure through a number of methods.
