What utter balls! They're talking out their arses.
As Alex said before........Nissan are well aware of these gearbox issues......... stay with the first dealer.........be calm and assertive and determine that they have infact asked Nissan's permission re warranty. Just because you let them investigate does not mean to say you are authorising any repairs. Again..as Alex said - do not under any circumstance authorise any work - sit tight and wait for a response from Nissan. If it comes to it then you could also make Nissan customer service aware of the situation yourself - but only as last resort. As you have quite rightly said yourself - your car is basically 'new' they can not wriggle out of the warranty on this one you just need to politley stand your ground.