1. It's explained in the Rage thread:
https://forums.alliedmods.net/showthread.php?t=179706
But it seems like it never made it out of Beta and I didn't really see people use it. It's potentially more powerful, but it's more difficult to use (create hook handlers that is). Orpheu is good enough.
2. In your amxmodx/configs/orpheu/, in functions and virtualFunctions folders. There's not a set list of functions you can hook, it's totally dynamic and you have to provide data in these folders for Orpheu to use. That's done using JSON files containing signatures or other data for functions. You can find many around the forums as examples.
3. Looks like CBasePlayer::AddPoints() is only used by CGameScore ("game_score" entity?). It works, but it's probably not called when you expect it to be.
5. According to ReGameDLL_CS, it looks like CS doesn't even support ShouldCollide API in the first place. I.e: it's never called.