1) Many modmods have an installer but for those that don't the files of the mod can be placed into the mod folder in the game (Be sure to keep a backup copy of the origional files)
2) More than one modmod can be used if it is Moddular, if they change different files, or if they do they can be merged using winmerge and a file editor such as notepad++.
3) I really don't know