I would almost point to the flywheel being the culprit. That inductor ring on the back is pretty flimsy in comparison to the weight of the flywheel and it could easily bend. The car is not going to start if there is a issue with the pickup from the crank sensor, just as a safety.
It is weird that it ran for a bit but maybe during its heat cycle something shifted? To do a clutch swap you don't touch any harness's other than the trans harness and unplug the O2s, both of which you could theoretically leave unplugged and the car would still start just fine.
Found this after a quick search on the internet, might be useful to see if a code is popping up on there.
2007-2010 G35 and G37 Sedan, and 2008+ G37 coupe diagnostics and settings display system