From my understanding of the Terms and Conditions of the Skywards program, you would need to first add the infant to the booking and subsequently upgrade the mum and infant together (infant at 10% of the miles).

The infant cannot be booked on a reward seat in J. I suppose one could book the infant in paid J, but they likely won't allow this.

So for 1, yea, it'll be 10% miles for the infant. For 2, I'm not sure whether they'll insist on canceling the upgrade before adding the infant, or whether they'll just do it at the J charge. It's worth calling them to ask.