Squirrels will kill adult birds at feeders also. When I lived around them I had squirrel baffles on the poles and all my feeders had grids. My platform feeder had a 1" grid over it also to keep out the starlings and cowbirds that bully the song birds. Keeping the feeders and birdhouses quite far from fences and trees helps also. Squirrels can jump really far. I have a friend who calls them flying rats. I also had a clear dome over one niger feeder. It's a lot of paraphernalia but I felt like I had to protect them if I was going to lure birds to the feeders. Here we have trouble with hawks that sit on the fence and wait for the quail and small birds to get distracted. There's kind of a downside to feeders and bird baths I guess. Birdhouses are easier to protect. Sounds like you have a good plan for the houses. So nice that you are looking out for the babies.

If it was in a cat's mouth, regardless of paint, Google "Avian rehabilitation" with your state and you'll get a list of rehabbers. Bring the bird there.

We have a house finch nest in our wreath one brood already hatched and fledged, then they were right back at it again. I see a lot of people who don’t notice a disturbance in the nest when the babies go missing. One thing it might be is crows, we had 4 babies and an unhatched egg and a crow came and took all but 2. It tried to come back, but I think we scared it off. We have video of it going to the nest on our doorbell cam. Crows are smart and like to observe, they will watch mama and papa going to the nest and also people checking on it as well. If you are checking on a nest be very aware of your surroundings

They take about 19 days to fledge from egg hatching onwards; you may want to try and figure out approximately how long the parents have been feeding them and that should give you an idea of how many days more to go. Pretty soon you should start to see some action