sorry man, know how frustrating it is to think you're getting her fixed and then finding out shit just got worse...
not really sure of the terminology you're using so correct me if I'm wrong, your engine is turning over but not cranking... or is it cranking and then immediately stalling(cutting off)? does make a difference.
based of the first assumption I can only point out a couple of things that probably won't have any affect, but might be worth a look at-
~ with my sentra, when the ECU went out, it would turnover but not crank. to make a long story short, I found out that the ECU has to be programmed with my VIN for it to work. not sure if this applies to your model but it'd be worth calling nissan to find out.
~ I've had several problems with the wiring in my sentra, from the headlights to my speaker wires in my trunk... anything you can check would be worth the effort.
other than that I can only point you to this factory service manual link for my sentra- http://www.nicoclub.com/FSM/Sentra/2002/em.pdf
would be better if you could find the FSM for your Xtrail