You do not need to keep asking permission to post something.. This is a forum for goodness sake, that's the whole point of a forum.
If you post something you shouldn't we will either tell you to change it, or just change it ourselves. If you post something pointless, like a tutorial on how to make custom license plates (which is something we already have) then we will probably remove it or close the thread (if its basically the same the same as an existing tutorial)
you CAN change to ALL cars even unfinished ones. But some unfinished cars will make the game crash because ... they are unfinished, Some will be drivable, but very badly.
The show all cars mod , will show all cars on the gps (all nearby players, police AND traffic cars)
Your brothers problem is very simple, he did NOT follow the instructions exactly as he should.. if he did it would be working
He probably forgot to move the .big and .map files out of his main game folder. When you unpack the archives you MUST move the original archives out of the main folder (You could just create a folder called "backup" in your main game folder and move all of the .big and .map files there..)
Leaving the .big and .map files (the archives) in the games main folder, when you have an unpacked game will ALWAYS make the game crash
I think the db_data.cpr is what you are looking for. I remember when i tried to decrypt and mod it, i saw all the carnames in it. But i didn't found out what to change. Maybe you get in contact with Xarlith or Mitkop if you like to learn more about it.