No none of the above issues are likely the problem. Certainly running in reverse will not cause that sort of failure. It may cause issues, but not that kind of failure. Although several types of boxes can happily run in either direction.
However the details we need to give you a more accurate idea of the problem are missing.
The first thing that would help would be the model of the box.
The drive shaft does not actually go all the way through the box. So it won't be the drive shaft. Do you know what part actually went through the box?
I also assume you still have fwd and revs gears working when the hole is in the box, seeing as you state that you only noticed an issue when you had noticed the oil
. Is that correct?
The actual driving load is seen on the "final drive" end of the box. There is a large Bearing that takes the fwd and revs thrust imposed on the box by the shaft. A hole at the front of the box is in the area's of the gears. They will have also have thrust bearings, but this is to take the thrust of the helical cut gears as the load tries to force them along the line of the helical cut. Normally these bearings are held in place by circlips. Unless you have a compleatly wrong box as far as Hp rating to the engine Hp rating, I doubt that the load the over pitched prop is placing on the box will be causing any issue's like you are experiancing. But then again, you can never say never in mechanics.
Yes the prop is overpitching. But just how much so is determind by the max RPM
unloaded minus the max RPM
loaded. The difference should be no more than 10%. 200RPM ruffly is a good mark. You may not operate the engine at full RPM all the time, but it has to be able to reach full RPM if you open it up fully.
If Pat(never monday) reads this, he may know of an advisory from Yanmar perhaps. But I doubt this has been a common issue. I have never heard of it before myself.