I feel your pain, but knowing a little more how it works might help.
The dealer doesn't pay for the repairs. Ultimately, Nissan pay for it. They will do that if the dealer determines that it's a fault in the vehicle or its components (and not just wear and tear or damage). That makes perfect sense. Further, below a certain $ limit, the dealer can do the work and bill Nissan, but above that limit, Nissan have to approve the work.
In order to make that decision - on whether or not the item is actually faulty, and it is actually NOT from wear and tear, Nissan require the dealer to inspect and determine the fault and how to resolve it.
It is absolutely reasonable for you to think that you can diagnose a problem and then just tell them that you need this particular part. Many of us here feel the same (and sometimes we can diagnose **** from the description offered here, when the dealers can't while they're looking at it).
But there's a process, and it must be followed. It's going to inconvenience people sometime and I'm sure the dealer is sympathetic but at the end of the day, they aren't being paid unless they follow the rules laid out for them by Nissan. Nissan aren't going to change the game because for the majority of customers, this is the way that works best.