I don't know exactly as I don't own the PC version yet, but since it's a Games for Windows Live game, you should be able to sign out of your online profile, go to create a new profile, then scroll down a bit in the text to find a link that says "create a local profile." You'll probably have to locate where the game stores your gamesave and move it to the new profile (and probably back it up as well, just in case), but again I can't test anything myself.
Hope that helps, though.
To play in offline mode and be able to save, without the need to keep disconnecting your internet, you will need to sign out of your usual (online) GFWL account. When it then asks you to sign into an account, you need to create a new profile.
Selecting a new profile will display a bunch of text highlighting the features of a GFWL account, scroll down, and you'll see some blue text prompting you to "create a local profile".
Click on that text and you'll be able to make an offline account that you can then sign into, save with, and use the trainer to your hearts content without affecting the experience of other players.
It was the dev's idea to force multiplayer and pvp. Not using that would mean that you don't play the game fully, and if you play with an offline profile you later won't be able to import it into your normal account, nor you can chat with your GFWL friends while playing.
So, since that forced MP was something the devs forced on us, **** it and enjoy it.
GFWL doesn't ban anyway.