OK, I think you have he resonance structures right, but you didn't put any charges in them.
I'd write them like this:
[ I-N=N+=N- <> I-N--N+(triple)N ]
As for the polarity of I-N, I think they are just after direction of the polarity. This is clear fron the relative electronegativities of I and N, and also from the second resonance structure above.
As for the mechanism, I would be tempted to go via a 3 membered iodinium ion intermediate (the same intermediate as in I2 addition), but I'm not sure about that, it's just the first thing that came to my head.