I'm thinking of the problem I had on mine either the BCM chip or the ignition switch.
I had the problem of sometimes the car would not start but after a few turns of the key the engine would come on, this went on for a couple of months when I came out of my work on the hottest day of the year in edinburgh and turned it over nothing, turned the key a few times, still nothing. Called out my friends at the RAC who know the car very well and towed it to Nissan.
Turns out it was a faulty ignition switch.
Might not be the same problem but I hope your car gets fixed .
I'm in the same boat about your garage they do their best for you even when it is the car but I have since fallen out with the garage and would not recommend them for buying a car. For the next service I have to take it 25 miles away to another Nissan Dealer to maintain the full Nissan Service History.