I'm 99 percent sure you did not damage the seal when you removed the clutch basket. I believe Scott's point applies to the K100 too, as well as the K75. It didn't leak before you removed the clutch basket so it wasn't leaking. I'm pretty sure it leaked from above the top of the lowest part of the seal, which it will if the oil level is above it when the basket is removed.
There is a photo which shows what I'm trying to explain, in a recent thread by Filmcamera.
Sometimes the problem is just that the clutch cable is stretched.
I think they're being over cautious in describing the clutch fasteners as single use. One of the reasons being that thread locker was probably used, so if heating them up before removing, and then cleaning the threads, it's not an issue to worry about. I think it's safe to reuse them, regardless. Next time you lube the splines, you can check to see if any are loose.
Since the clutch basket is removed anyway, you might as well replace the seal and O-ring, and you probably won't have to go back in there for many years.