I now have both axles back on and my memory isn't photographic anymore, but from what I can see through the transmission oil filler hole, the ring gear is sandwiched between the input pinion and the ring gear carrier. Can you confirm this? If so, the 12 bolts/rivets don't see any spiral/bevel thrust load. The tension on the bolts would be just from initial tightening. Oh, I suppose there would be a little fastener tension from the "oil canning" effect of the pinion's side force on the ring gear. So shear is the primary loading on the fasteners, and I think Mike is correct, if one bolt sheared, they would all shear. The broken bolt found in the bottom of the case looks like it failed in tension. I suspect over torquing upon installation.
Thus, unless I'm missing something, It seems to me I'm not taking much risk when re-assembling with one of the 12 fasteners missing. Besides, it looks to me like the only way to replace the bolt would to remove the engine - an exercise I'm not anxious to go through un-necessarily. I guess that would expose the open back end of the trans-axle. Right? Anybody know of a good cross section illustration of the G drivetrain? The manual sure isn't good for that.
I've only had the tractor 18 months and have run it very little and, as mentioned earlier, don't expect it to see anything but parade, grand kid and tractor ride duty. I have no idea as to how long the bolt has been broken/missing. The tractor has experienced at least one broken axle that was welded before I got it. Who knows when, but maybe it was at that time that the bolt broke.