It's the same as with a bespoke vs MTM suit. Your own pattern is made with bespoke, whilst a pattern is altered for MTM. A bespoke collar can be made for you whilst with MTM you have to pick an existing collar. I'm pretty sure with MTM I couldn't have my own collar and cuff designs and adjust things like the placket width.
Midnight blue is just as classic and is looks blacker than black (which looks kind of green in certain lighting), so there's no reason why he shouldn't go with midnight blue. The jacket and trousers need to match and will be sold together. A perfect cannot be bought separately.
There is no rule that the bow tie and cummerbund must match, but when they don't match the bow tie has to be black, or dark enough midnight blue that is looks black. If it's that dark it needs to match the cummerbund. So go with the black pre-tied.
This is good advice. To expand on it:The wing collar shirt mentioned above is the only one that should take single link cuffs. This shirt also must take studs, if that wasn't a given. It's a white tie shirt and what was originally worn with black tie until soft shirts started to become popular in the 1930s. By the 1940s it wasn't as typical to wear this type of shirt with black tie. I find this type of shirt too fussy for black tie and a wing collar shirt to be more like...
I don't appreciate the sarcasm. All I can say is that people in the US and in the UK don't call single cuffs simply "link cuffs." Any other people I've talked to outside of those two countries has been only online.