Would I still have a gap on the NISMO?? Are the gaps larger on Pioneers vs. Kenwoods?

Yes, the Nismo has the same panel as all the other non-Bose models. As far as other Pioneers they might vary from series to series. The OP (and I) have the AVIC-X920BT. There are others on the board with Kenwoods. Go look at AK's write up in his signature.

I've heard that the gap is more prominent with Pioneers and less noticeable with Kenwoods and Alpines.

How is the rear camera mounted beside the plate light, is it attached with screws/bolts or something else?

The camera comes with a mounting bracket which has an adhesive stripe on it. The advhesive is very strong and once it's mounted it will not come off.