Yes, The mother bird will usually abandon the nest if human scent is present.
Not exactly. She may abandon it or push it out of the nest because of the smell, if you only touched a baby bird, then you can put it back, but if you held it in your hands, then you probably left a lot your scent on him\her. Baby birds like crushed up worms that they can easily chew. Add a little bit of water, and it will open it's beak when you gently tap it with a stick. It will need to eat every 2-3 hours. You'll need to take care of it, so good luck!
No, it will still take care of the baby bird. I found some birds on my porch and before I touched them I went to google to see if the mom bird would come back to her nests if I messed with the birds and they will. I would not interfere too much though because it might spook the mom bird off.
It probably wont kill it but it may abandon the baby.