Out of curiosity I tried everything you asked. I disabled steam cloud for FNV, switched to offline mode AND disconnected from the internet. The game ran fine, same as always. I loaded up my latest save, generated a new quicksave and autosave.. tested them.. they both loaded fine. I then exited the game and restarted, whereupon same as always the new quick/auto saves are gone and replaced with very old ones. I tried deleting my quick/auto saves and .baks and reloading and the old ones magically reappeared this way as well.
Thanks, I am at work so I am testing vicariously through you.
I do know that the Steam backups are stored locally and until I deleted it it kept re-appearing (note I never Autosave and disable those features). Also, during the day yesterday, I kept getting my manual saves backed up on me too until I told the game to play Off Line. However last night I did not have any of these issues.
Do a search in the Steam directory and look for Autsave (and or Quicksave) and delete what you find in there that pertains to Fallout New Vegas. Then try it again please