All things being equal, no. A single trussrod will be sufficient at the scale lengths you're looking at. If you use unstable woods in your neck (heavily figured timber, spalted timber, timber with low inherent strength) or are going for something really thin then consider adding reinforcement.
No rules, just whatever suits the application of the completed instrument, be that playing style, client requests or comfort/feel. I personally wouldn't exceed more than about 1.5" fan difference from top to bottom, as I feel that extreme fans must make certain hand positions quite uncomfortable and negate the benefits of multiscale construction.
Not sure what black nickel looks like. Is that anything like Gotoh's Cosmo Black?
Multiscale builds already require the use esoteric components, so choice of finish will be limited even further beyond black, chrome or gold.
Technology for Musicians make individual saddles, but only in the three standard finishes.
I've seen some people use Wilkinson saddles on a custom baseplate to make up mutiscale bridge assemblies.
Fanned Fret Innovations make individual saddles, but only in chrome and black.
Rondo Music occasionally sell their individual saddle assemblies as spare parts. Quality may be questionable at the prices they charge.
No idea. I've only ever done a slanted nut. Visually I think it looks better than a straight nut behind a slanted zero fret, but again that's down to personal aesthetics.