You are definitely over thinking this. Nobody has ever been rejected by McKinsey because of the suit or tie they wore (unless presumably someone went completely off the books) To answer your questions:

1) Any conservative colour will be fine. More important is the fit of the suit - don't wear a suit that is comically big on you, you have to look sharp

2) This does not matter and is just a matter of preference - on the job it's about 50/50 cufflinks vs no cufflinks

3) Any tie (just try make it fit the suit and shirt

4) As any suit-lover will tell you, the answer to this question depends entirely on the suit you wear.

5) Wear a watch if you normally wear a watch. Would probably refrain from wearing anything too flashy (e.g. a gold rolex). Benefit of watch of course is you can monitor time between interviews.

6) Up to you - if you normally wear a pocket square, go ahead, otherwise don't worry about it. I have never worn a pocket square to the office, only to drink events/ office parties.

Don't stress about your clothes, focus on the interviews!!