First and fourth problem are simple. Second difficult and three also very difficult, and it wouldn't make sense for me to try to explain those.
You can change pokemon by modifying their stats. You can do this via hex editor or using some utilities available. Look for these in Skeetendo Inc. (hax.iimarckus.org).
Evolution methods for pokemon are such that you can easily edit with a hex editor. In order to learn how to for example manage to edit evolution methods of pokemon via hex editor, you could start off by following my rom hacking tutorial videos:
https://www.youtube.com/watch?v=P55prFTaZoE
Part 2 hands over all the information you need in order to edit the evolutions and much more. But in order to understand it, you should watch part 1 first.
For pokecrystal disassembly, you should start at by simply reading the README.md here:
https://github.com/pret/pokecrystal
It covers installation guide and documentation (along with wiki which has also plenty of information). Joining #pret discord channel may also be useful.