It is 1.5 % times the full Y fare. Hence full fare Y * 0.015 per kg. Hence if 140 USD was full fare, it's 2.10 USD per kg or 42 USD per additional bag. (Or do you want to check an additional 100 kg? )

You can combine your baggage allowance just like on any other weight concept carrier. I.e. 4 people have 80 kg and you can check as many bags as you want.

CX has its own rules (they're HK based). CZ and MU rules are the same.

Don't worry about the carry-on, two or even three pieces are ok.

BTW, the 1.5 % rule is the standard IATA rule for all airlines that do not explicitly apply other rules.