You've probably deleted something vital by accident or put the mod in the wrong place. It's always a good idea to make a few copies of your minecraft.jar if you're going to meddle with it. Need help downloading mods? There's plenty of very helpful videos on YouTube that can give you a hand. :)
If your minecraft's broken, maybe ask in the Minecraft forums for some help getting it back :)
The Server has to install the mods Not You.
They don't.
Minecraft 1.4.6 mods will work with 1.4.7 sense they are both a 1.4.
It is quite possible that your are installing the mods incorrectly or attempting to install incompatible mods To avoid this use a mod installer (like MCPatcher) and install Forge Mod Loader first so all your mods work together
NPCs were added in 1.8 you now don't need mods!
You need to install Minecraft Forge in order for many mods to work. Look up a YouTube tutorial to find out how to install Forge.
no mods dont work
No
No your have to wait for the mods to update which may take some time.
This is a question to ask on the My Minecraft Mods forum topic. It may not be compatible with a Mac yet.
Re instull Minecraft
The Vanilla version of Minecraft refers to the game with no mods or texture packs installed.