Yep, the snails and sally lightfoot crabs like to go back and forth, lots of food back there, a fish does end up back there on occasion but not often. The drains are screened so nothing can get into them, there is no dead space, the screens go from top to bottom so there is full water flow, there is no detritus in the right and left sides at all, it does accumulate around the drains but can be siphoned out or just stirred up and it will go down the drain and into filter socks.
As far as the size if I remember the original BeanAnimal designs were on smaller tanks, I did a 24" with a 3-4" overflow, as the design got bigger and flow increased in the larger tanks I increased the size/depth, the water level is about 3.5" above the siphon drain or it will suck air and about 1" below the weir line to reduce the water noise. Another good reason is just more water, the overflow holds 14 gallons, good place for macro Algae, etc.
If I were to do another tank I would do the overflow exactly the same as this one, there are a few other things I would do different but I very happy with this design