When I referred the command line was also for you to try to mount the file not just the device.
Have you looked at growisofs man page to find a way to try to close the DVD?
What's the version of dvd+rw-tools package you have with K3b is it 0.19 or 0.21?
0.19 seems to have a bug causing problems to close the discs, that was corrected by 0.21.
K3b 0.12.12 was released just a few days ago, you could try to install it and see if it can solve something for you:
http://www.k3b.org/
Good luck