Skyrim character progress and Steam Family Sharing

Giving yourself items should never break the game (unless you give yourself quest items, but they are useless, so why would you?). Advancing quests with the console can be problematic (and, in the case of some quests, impossible) both because there are undocumented stages to most quests that can make important changes, and because things other than quest stages may be involved in the quest, such as opening passages, killing characters or acquiring items.

Overall I would recommend making a new character and playing the storylines you haven't played with your last character, then deciding whether you want to risk messing up your save or if you want to replay the old storylines again.

As Scott Weldon said, there doesn't seem to be any way to do this other than disabling sharing of the friend whose library is in use, or using offline mode as Jaxir said to prevent the library from being in use in the first place. You can change the order in which your friends share libraries with you by having them unshare and reshare the library with you, in order to play your games with the library of whichever friend is least likely to be using it, but for a general solution to choose which friend's library to use, you'll need sfs-select.

SFS-select is a python tool to enable and disable friends' shared libraries by modifying the local cache remembering which accounts have authorized you. It's a bit of a clunky workaround, but it lets you pick exactly which friend's library you would like to use for a shared game, without needing your friends to log in to your computer again, or changing the actual permissions registered on the Steam server at all. It includes both graphical and command-line versions. You can download it here or read about it on the Steam forum thread.