I do agree that that the studs are most likely plenty to hold up to the stress of mounting the control arms in this way. You definitely make a point when saying alot of cars use a rear subframe to hold both the differential and the control arms. However one thing I also acknowledge is that the s30 was never designed for this. I think this could work well, but it will need plenty of added bracing around this area. I think some framerail reinforcement and better mustache bar bolts may be required for good longevity.
I do feel though as this falls apart by the way it is bushed. The differential in a high horsepower car is going to move around and flex quite a bit no matter what unless its mounted with about 500 lbs of concrete. The reason why the subframe in a 240sx can mount both control arms and the diff is that it is completely isolated with correctly implemented bushings. That alows it to flex without affecting the geometry of the suspension.
In the senario of the apex kit there are many differences. Since the rear of the differential is mostly hard mounted and the mustache bar is bushed, instead of the diff bushings absorbing the rotational forces, the mustache bar bushings will absorb it instead. Since it is the source of mounting for the control arms, the entire thing will twist left or right on hard acceleration. I think this would probably cause a skew of tow in on one side and tow out on the other, but i'm not entirely sure.