If you don't mind me budging in-
For the most part, NWN mods don't alter the official campaign. Â Almost all of them are, as you said, 'quest packs'- totally new mods that have no ties to the official campaigns what-so-ever (like the Witch's Wake). Â Some of them are the worst of the worst, and some are much better then at least the NWN basic official campaign. Â Check through the Hall of Fame if you want more of them that are decent (not to say that everything there is decent- but most of it is
).
Installing things for the official campaign is done in two ways- through assosiated hak paks and override folder. Â Unlike BG, it's generally a good idea to put nothing in your override folder; bioware strongly recommends against it. Â Putting stuff in the override folder has crashed games, screwed up patches, messed things up to the point of needing a reinstall, and so on. Â It also has done nothing but helped someone enjoy the game (I know someone who went and overrided all the music files in the game with Nightwish and Blind Guardian. Â Whatever makes you happy, I guess.)- just make sure you know what you're doing first.
Hak paks basically hold the custom content- they're usually paired up with the 'quest mods' (actually, let's just call them 'modules'; anything else is a 'modification' or something, <i>yeah</i>). Â If someone who makes their module wants a monster that's not included in the official toolset to be there, they can do it through a hak pak. Â Hak pak's on there own will not mess up anything- they can sit and dance in your hak pak folder forever. Â When you open things up in the toolset and apply it to a module (or the offical campaign), bad things (or good!) can happen then. Â It can overwrite things already in the module, basically. Â In general, it's not a good idea to apply a hak pack to anything unless specifically told to.
(I'm sorry if what I'm saying seems obvious.)
The only thing that comes to mind that you may be interested in is the PRC, which if assosiated with a module or the official campaign allows you to play with more prestiage classes. Â Some of them are insanely overpowered, and some of them don't fit the story at all- however, if you fiddle with it, you might be able to find one that you like and seems interesting. Â Obviously, it's not supported by Bioware, and no one will respond to your prestiage class (and no special items for you, either)- but it's interesting enough.
http://nwnprc.netgamers.co.uk/I don't really know of anything along the line of cleric remix, song and silence, and so on (unfortunately). Â Almost everything in the engine is hardcoded, and people generally want to make their own modules, not fiddle around with others. Â I know there have been various attempts to turn things in to DnD 3.5 and to add new weapons and basic classes- I've also never heard of any of that being completed.
For installing- .mod files go in the module folder, .hak files go in the hakpak folder, .bmu files go in the music folder, .tlk files go in the custom talk folder and .bik files go in the movie folder. Â If something needs to go in to the override folder, you'll usually be told.
For uninstalling, simply delete the files. Â There's no easier way. Â Just make sure if it's a .hakpak or .tlk or whatever that you've went and unassosiated it from anything you still want to keep around (IE, if you added the PRC to the official campaign and want to uninstall the PRC, make sure it's not still locked to the offical campaign, as if it is, you won't be able to run it).
Sorry for the long post.