If you can play Minecraft then you can install the Too Many Items mod.
I am unsure if a mod like this exists, but if it does then yes, you will need to use this mod.
The latest version of the Aether mod is for 1.0.0
If you install a mod it will work in all gamemodes.
Yes, garry's mod will work with portal and you will have all the HL2 stuff.
There is a timber mod for multiplayer that the owner has to put on, so yes it does work if you're the owner.
It depends which mod you are using. There are many different mods that add Herobrine into Minecraft, you'd have to look at each specific mod to see.
I would use a mod called Too many items too, I have a tutorial for how to install the mod. (See "Sources and related links" below) Too many items is a ingame inventory editor.
Garry's Mod is only for Valve-released games, so no.
There is no mod for it. But in the animus desktop (the pause menu) you can go to options > stats > fight and you can see how many people u killed.
As of right now, too many items doesn't work. Because the mod creator needs to patch it to work with 1.0.0.
Yes