Sorry but what a load of rubbish that it needs the dealer principle to authorize a courtesy car. This sounds like a very poor Nissan dealership. I took mine to Holdcrofts in Stoke last month for some warrranty work and was given the option of having a courtesy car by the Lady on the service desk!! It is poor that you need to wait for these parts for such a long time, Holdcrofts needed some parts for mine and got them over night!!
You need to kick some ass.....i know i would (BIG STYLE).
And final note i know it's not nice having this happen and i am sure your car's failiure is a one off, but for one i would not be getting rid