Booked IAH to NCL (return) using maximum Avios (180k) and GBP 450. I'm about to call to add an infant :

1. what will I be charged? Avios and taxes? Or 10% of the cost of an adult ticket?

2. After reading some helpful advice on this forum, I think I am cutting it short for the Heathrow connection on the return. Picked what suited me not the little one! Lands from NCL at 13.25 and departs for USA at 15.05. When I call to add the infant should I ask to take the earlier flight from NCL? And what will I be charged? Or should I just ask for help to make the conection?

Oh and thanks for the useful update re NCL !

Thank you