Hi there,
So I finally had time for further tests, and before going any further, I tried making a TM backup of my macbook air using exactly the same method...
And well, it worked flawlessly.
So looks like the problem is on the imac side and not on the NAS side...
If you haven't done so update to 10.8.3, this might solve the problem, I still think it's unmounting the shares while backing up and Kernel panics.