Samurai Shodown actually has limited color support, so that's normal.
You can't fake a NGPC cart, so it basically comes down to if the manuals and inserts are real. In all of my years of collecting, I have never seen a bootleg that wasn't larger than a normal cart. The label could be fake, but there's basically zero chance any of the carts themselves are bootlegs. Regarding the double label, I haven't seen that before, but my experience with the black and white games is limited. SNK did this with MVS carts all the time (and I believe a few home carts as well), so it wouldn't surprise me if they did it with NGP.
The Neo Mystery Bonus is real because that game is not worth much.
I've never seen a loose NGC 98 Plus before (as turq said, they typically show up complete about once a year on ebay). I find it very hard to believe he somehow ended up with loose ones for which he could make fake packaging. Unlike every other NGP/C game, there is no Japanese version that could be converted to a bootleg.