Once you click the shutter = put in tangible form
You can't copyright something already in public domain. I'm not sure about "improvements". Too bad we can't ask Andy Warhol. You'd probably need a copyright attorney for an interpretation given your specific scenerio because your questions are really different and very specific. And remember, an attorney gives you an interpretation not a guarantee. Anyone can still sue you for any reason at all. They may win or may not, but the law is always interpreted.
Yes you are allowed to profit from works in public domain...but so is everyone else.
Now, let's call a spade a spade. You want to sell these photos, possibly in a collection. Are you wanting to make sure you don't get sued or make sure someone doesn't steal them? Both? If you are serious and are planning on making a profit then consult an attorney. BTW, I think it's great. I'd rather see you publish and profit than lose them forever.