My setup is very similar to yours with the c2c overflow and the existing holes in the back of the glass tank. Just wanted to add my personal experience to explain why the third dry pipe is so important:
Two years ago we took a two week vacation to alaska. About a week into the trip i got a call from my buddy who we had taking care of our dog and cat. Friend, Hey dude your tank is making a ton of noise, water slurping and gurguling sounds. Me, any water spilling? Friend, No everything in the tank looks normal. Me, Ok well if you could keep checking on it when you let out the dog and if you notice any water just pull the power cord for the main return pump in the garage. My buddy has had his own fish tanks over the years so he knows what a return pump is and how to unplug it. I didn't feel like explaining how to turn it off via the neptune apex, and my cell coverage in alaska didn't really allow me to remotely do it with any consistency.
We return from vacation, the tank is making all of the noises he described and i figured it must be something stuck in the full siphon at the gate valve, it had happened in the past. No big deal open up the gate valve and it will clear out, reset the gate valve back to normal. Nope, not the case. So I inspect my overflow box and see the water very high using all three pipes. Well thats new, turns out a turbo snail made its way into the durso pipe and got stuck at the joint of the bulkhead and lower drain line, effectively clogging the secondary drain and eventually the water rose high enough to use the emergency.
I have pictures of the snail in the bulkhead somewhere but couldn't seem to find it for proof. I have a similar egg crate catch to yours and thought for sure no snails could get where it had gotten. I have had numerous fish make the journey through my plumbing and enough small snails and hermits in my sump to know that it is very likely to happen again at some point. I have even had a fish make it from my display to my sump, through the return pump to my refugium twice. Things that you may think are impossible are very possible and probable in the aquarium.
Somewhat unrelated but just to make the point, here is a pic i did find of my 8" rose bubble tip nem working its way through a 3/4" lock line with a flat fan nozzle on the end. It made its way from my fuge, under, over, under a bubble trap, through a 1" drain with gate valve that siphons back to my display.
If i were in your position i would recommend doing something like i did. Use the two existing holes to go to a overflow box mounted on the back of the tank. Then use the full bean animal overflow design. I just welded together an acrylic box and drilled the side to match the holes in the tank and the bottom for the bean animal drain. Here is a picture of how i did it. The box doesn't really take any more space behind the tank than the plumbing you have.