A: Yes. The RAGEMP launcher can sit on your C: drive, but the game path must point to where GTA V lives. You cannot separate the RAGEMP client data from the game, but the launcher executable can be anywhere.
Click and navigate to your main Grand Theft Auto V folder. A: Yes
The error dialog is actually a blessing in disguise. It gives you a text box and a button. Here is exactly what you need to do. A: Yes