The hole goes to the tensioner actuator rod. There's no oil pressure in there, and a pretty long thin path to the inside of the cases. I would say that oil leakage is not a serious problem here.
As for getting the stub out: opening the cases is not going to help, you have to attack it from the outside (been there, done that). A reverse drill bit is likely the best approach, the easy-out that fits in a 6mm bolt is really small and delicate: once you snap off the hardened steel easy-out you have more trouble that you started with since you can't drill it out. The bolt itself is soft steel and drills easily, keeping the drill centred and straight is the problem. The hole and bolt are odd, the bolt has a 6mm part at the end that actually screws into the case, and an 8mm thread close to the head where the locking nut goes. The case hole has a bore to clearf that 8mm thread, I made a drill guide by machining a steel rod to fit snug in the bore then drilled a smaller hole through to guide a drill roughly the size of the thread root. I have this kit somewhere but can't find it now...
