Anywhere below your bioballs will be fine. Where you have the hole will work. Remember your water level in your Wet-Dry(as long as the water goes to it first-as in my diagram) will always be the same, you will see no fluctuations at all in it. Your fuge is where the water levels will fluctuate if you don't have an ATO. Specifically the return pump chamber first.
the bulkheads will work fine IF he has the room to do dual levels under his tank. if there's not enough room, the utube would be necessary.
I disagree, it wouldn't make a difference at all. Your drawing has two tanks as well. As long as they fit under the stand, it won't make a difference at all if you use bulkheads or the U-tube. Cooqie can put each tank right next to each other, and as long as one of the tanks is raised(i.e. the wet-dry) it will work perfect. Drilling bulkheads would be a much better/safer option than the U-tube IMHO. All you have to do is drill a couple bulkheads in the wet-dry, then put a couple 90 degree PVC couplers pointing down to drain into the fuge.