LOL, i just wanted to say that good or bad, that is quite impressive!

I've set up a bunch of tanks from large to small with TBS rock. Some rock that was fairly new/young and other rock that had been down there for many years. Small pieces to almost boulder sized. I've had a range of critters, the worst was a large stone crab. I've had my share of mantis, but even on the biggest tank with the oldest rock, i still only had 3. Never even knew there were octopus!
I'm sorry I can't answer your question since I've never been in this situation but wow. Super interesting. Hopefully you can at least knock down the mantis population with some suggestions here. Whelks you can pull out when you see them, same for gorillas, isopods will age out rather quickly. I have a mantis in my current tank, just the one as far as i've seen, and in the more than 1 year that I've had the tank running, this is the first time that a mantis hasn't come fully out looking for food, allowing me to grab him. That little sucker is just continuing to enjoy the free escargot!