New to modding

Im still a little uncertain about some of this, apparently, and messed up my install which im reinstalling now. After the unpacking is there any way to make the game stop reading from 'data' and revert to the normal files?

Is the unpacking necessary to install a mod? What if the mod requires that you overwrite some of the unpacked files, you have to manually back them up and replace them if you want to install another mod?

And how to do you tell if the mod is correctly installed if nothing visual is immediately obvious? Thanks.