I'm glad to hear she's doing better.
If she's prolapsing, then she's straining to poop. I would make sure she's getting more fluids. Offer her wet mash to eat for a while instead of dry crumbles. I would also give her a little coconut oil or mineral oil (to eat).
Photos of the vent...
I'm sorry about the chick:(
I think you are doing all you can.
Keeping her hydrated is important. You can try giving her a direct oral dose of Poultry Nutri-Drench (2-3 drops) twice a day instead of diluting it to see if that helps.
Any solids coming out at all? What's her crop like?