1) For C, I'd go with NH. I believe their new seat is the flatest of those choices. The others, while still lie flat are also angled more. Unfortunately, NH C is also the hardest to find for an award out of those airlines.

2) You cancel the Japan ticket for a £25 charge and book your Latin America ticket. If you don't have enough miles to cover both, you'll have to wait for your Japan miles to re-deposit. You can't change one for the other.