If you prefer to perform the maintenance tasks manually, which are done for you with the full Tune-up, then OneCare isn't really the appropriate solution for you. You can set Tune-up to run monthly and then perform whatever else you wish on your own schedule outside of OneCare.
Note that you aren't actually running Defrag "all over again" since OneCare does a timed execution, meaning it will not check the drive first and it runs until done or time is up, whichever comes first. You are correct, though, that if one part fails, the whole tune-up is considered to have failed. If you are looking at the yellow message in the report about Defrag not completing due to high fragmentation, ignore it.
-steve