How hard would it be to get rid of the elevator and replace it with a flight of stairs?
If you did a simple flight of stairs it would be easy (such as the one in the tower of Hendrix). All you would need to know is how to delete stuff, add blocks and if you wanted to add a door you would need to know that. I think compiling the map would take longer to do. Iv done a very simple map before and I didn't find it all that hard I just couldn't compile it.
That has never happened to me. Did you try re installing Hammer?
Depends if the BSP has something to stop editing, you can remove it by removing either any and all lights or env_cubemaps and readding them.
If not, then it might just be you, if you had the .VMF, I could compile it myself.