1. Not sure. Possibly use it to hook stuff that can't be hooked otherwise by Orpheu because some data types are not supported. It should be possible to go around that stuff in Orpheu with some memory hacking though. You should just stick with Orpheu and not bang your head thinking about Rage.
2. InstallGameRules is a function in the game dll that gets called when a g_pGameRules global variable is to be initialized in the game.
HL version of this function (each mod can have its own, but they all return a CGameRules pointer):
https://mxr.alliedmods.net/hlsdk/sou...erules.cpp#311
It's just an easy way of retrieving g_pGameRules which can be used to retrieve all kinds of game data or call some methods from CGameRules or CHalfLifeMultiplay classes. New 1.8.3 get_gamerules_* natives are dealing with this g_pGameRules.
3. When do you expect it to be called? As I said, it looks like it's only called by game_score entity and not when for example someone gets a frag by killing other player.
5. I believe ReGameDLL_CS accurately depicts what Valve's CS code is. I'll maybe take a deeper look into this.