Short answers to all of your questions.... MOD's that you dl form this site are contained in a .pk3 file, in which you insert into the main folder of your MOH game. You will have to try out each mod to learn the affects and advantages of the mod (unless its listed in the readme), and if you dont like the mod after youve tried it, all you have to do is delete the .pk3 file that you put into the MOH folder, and the mod will be completely removed form you game, and it will be turned back to the origional state that it was in prior to using the mod
DL a bunch of mods, and try a few out, if ya dont like em, just delete em. As simple as that. A good mod can really make the game more enjoyable.
Have fun with your mod, and hope this advice helps
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~Blaze