I also have a leaky slave…my hydraulic fluid container has got progressively darker over the last few months; the oil level is still the same though! From what I have read in the other forums, this appears to be a common problem and not just with KTM. Anyhow, I need to get around replacing the slave O-ring then the fluid, once I've decided how to proceed with my shaft problem.
I doubt such a tiny amount of hydraulic fluid being constantly mixed into fresh oil could attack a bearing or shaft for that matter, any fluid experts out there? I'm more inclined to believing you had a random catastrophic bearing failure, but here are some points to ponder over…
1. If the slave does leak fluid to the engine and the fluid does attack bearings or shaft metal, it would begin leaking into the engine on the LHS of the engine close to the LHS main-shaft bearing, causing mayhem there… Have you split your case yet to see the state of things on the LHS?
2. Looking at the exploded diagram of a 690 LC4, hydraulic fluid cannot end up directly at the RHS main-shaft bearing. Any fluid leftover from the LHS heading towards the clutch side would have to work its way through the main-shaft and in so doing would end up being dumped inside the clutch via the pressure piece/bearing. At the speed the clutch turns any hydraulic fluid IMO would probably get flung all over the place under the clutch cover, mixing well into the oil if they are miscible. Even if the bike was not running and fluid seeped out from the slave through the shaft, it would drip from the clutch rather that move inwards towards the bearing.
3. If the fluid was the cause in my case, why are all my visible contact surfaces so clean? For instance, at the needle bearing i should seen more wear where the needles contact the shaft compared to where there is no contact, yet, the shaft surface is clean and even from the main-shaft bearing all the way to the inner clutch hub mating area. Similar observation on the clutch cage needle bearing mating surface, clean and even.
4. Lastly, if hydraulic fluid can attack bearings, then the bearing failure should affect many more people and not just KTM users seen as the leaky slave is a common moto issue. Unless it’s down to the specially silver coated bearings, but then many more KTM owners would have the problem.
Considering these four points, I really can't associate my problem with a hydraulic fluid leak. If anything my bearing is bad and caught occasionally, eventually wearing the shaft, but that would have to happen often and I would then expect to see a bright blue bearing yet it appears as fresh as. Furthermore, with the clutch removed the main-shaft bearing spins ever so smoothly.
The fact my main-shaft slides so smoothly in and out of the bearing means either the two bearings or both sides of the shaft (more probable) are worn. I may just have a naff shaft they forgot to harden or perhaps it's the other way around the the bearing were not properly hardened, but both is a bit farfetched! I'm so tempted to leave it as is and change only the needle bearing which has some radial play, rather than have to tear down the whole engine. Big risk though, especially when I see the pics in your post.
BTW, no markings whatsoever on my clutch cover, the shaft doesn't move out far enough, yet, for the basket to touch it. Would love to have a clear clutch cover, that would be an awesome Powerpart and let you keep an eye on goings on inside