If it's heavily planted, you could turn them loose in a couple of weeks. They are pretty good at hiding in the weeds. The adult platies will most likely eat them given the opportunity, but since they mostly stay near the top and the cories will stay mostly at the bottom you will probably be fine letting them out whenever you feel like it. You could PROBABLY let them out now but it depends on what you want out of this batch. I used to stress over every single fry death, but I have so many cories now that while I still try to get as many as I can to adulthood, I don't stress so much when some of them don't make it. If you are determined to get absolute maximum survival rate, then leave them in there for a while.
As far as feeding them, I do pretty much what you said. I dip a fingertip in the water and then in the bag and then back in the water. What stuck to the fingertip is how much they get fed. Some of it does go right through the mesh, but it isn't so much that it will foul the water if it goes uneaten and/or gets sucked up into the filter. I also keep moss and baby anubias in the breeder net all the time so I also think/hope that there is plenty of stuff in there for them to nibble on as well. I do turn the filter output for a little while while feeding or it will flush all of it right out the bottom as soon as it settles. Honestly I'd be surprised if half of what I feed is actually consumed, but for me it's a small price to pay to keep my "herd". Everyone in the house and all the family that come over all love to see the cories. All the kids/grandkids want their own and I tell them when they can take care of a tank I'll give them some of the "herd" for their own.