The main reason they don't allow you to redo them is people could abuse the extra exp you gain from doing a leve. If you could constantly do 90% of a leve and then abandon it somehow then you'd end up getting constantly high exp rates even on solo, which is something SE want to avoid. They want people to party up for that kind of benefit. Still, if you log off and log on again I think it should keep your timer going as if you never left, personally.