You are an INCREDIBLE Mother - you SAVED HER LIFE, my dear! You truly did. You gave her breath again and did everything correct in a crisis. Please
do not beat yourself up.
It sounds like it was a combo of a couple different things, neither of which were anyone's fault. These babies are very strong and can heal very quickly - so try to stay positive and know that we're pulling for your baby.
My Marcel was very tiny at 16 wks as well (1lb 4oz) - his growth spurts were later (as seems typical of some tinies) and he is now 3lbs and healthy. One thing we did is put a collar w/ a bell on it so we could hear him at all times. Of course, we never use/used that collar for leashing (harness for that). When he was little, we took the collar off whenever we were not there - and put it on as soon as we were home. He now wears his collar 24/7 (as does Wylie) with his bell and I.D. tag. To properly fit a collar for a tiny, you really only want to fit a pinky-tip btwn the collar and her neck - you do not want it too loose as that's unsafe.
I hope you update with good news. ALL the best to little Gia.