im having this same problem, and same questions as the original poster. after studying this post carefully and whats in my backup archive, i came to a few conclusions that hopefully help others, and i hope somebody can step up with some hard evidence so we can solve this.
i want to delete the first few backups in a list of 6 of them as listed in the windows 7 ultimate backup tool in the action center.
i have no guarantee that these are incremental. furthermore, maybe im giving m$ too much credit but one would think that an idiot proof system that clearly says "deleting backups will allow to free up some space. All files during the selected period will eb deleted." means that your safe to delete the oldest backup without losing any files other than what you have already physically deleted from the system since that backup date. This would mean that the backups listed ARE NOT incremental backup parts of a whole, but are essentially separate complete duplicate archives each for a specific era.
This means its safe to delete one of them from what i gather. whats the use of them if not?
More info to corroborate this is the fact that all 6 of the backups listed in my psersonal list on one of my machines are 17.5-20.5GB a piece, taken in 3 month intervals, which means that these are probably full sets incrementally backed each, completed and finalized at the end of each era, then when a new backup is created, the process starts over as mentioned in previous posts. so it appears each backup is a complete backup of the era.
So it appears that when you delete a "backup", you are deleting the entire interval, Each of the rest of the remaining backups, or intervals, are "COMPLETE" backups as well, which means that data that was BOTH touched, and untouched, is still in each of the rest of the remaining backups which themselves are normally primarily duplicates. So if i guess right, i could delete ALL backups except for the very last one and im good.
Logically this would be an idiot proof system, and combined with the fact that i came to this conclusion after a bit of research, i would guess one can delete all backups except for the system image, and last backup each time there's a new interval in the list if more available hard drive space was desired to be freed up.