Hi there. The mom platys red belly is more to do with her colouring, than the fact she just gave birth. I have never had a platy that has given birth have a red belly. Babies are very clever at hiding so there may be a fair few who are very clever.
I use a spoon to crush flakes into a fine powder. Try to sprinkle in the areas where you have plenty of hiding places as they will then manage to feed, and stay hidden from any hungry adults. However, I find that platys will eat any and all babies even straight after they are born, and as she has birthed them in the main tank I would imagine the majority will have been consumed. So this baby is a very lucky fella/lady. Keep an eye out at water changing times, as I have found they come out of hiding during water changes and be ready with a net. I wouldn't say she still looks pregnant, but she still could have a few more in there. Maybe putting her in a hatcher for a few hours to monitor any progress she may be yet to make. Good luck