Your returns should be placed slightly below the display surface level so with a very small, easily calculated, amount of backflow they are exposed to atmosphere and the siphon breaks. Since water cannot jump uphill they cannot possibly cause a flood. You always maintain a small amount of freeboard or spare capacity in your sump to contain this small amount of water. Most skimmers work best with say 8-9" of depth anyway so you should already have romm in the sump.
In my case, I have a 100G display which is 60" long by 18" front to back and my returns are placed 3/4" below the surface. 60"x18"x0.75"/231 = 3.5 gallons of backflow worst case. Once the level drops 3/4" one or both returns is exposed and the siphon stops guaranteed, it cannot continue so the flow ceases to the sump.
So my 30G sump needs to have 3.5 gallons of freeboard at all times. My skimmer sits in 8" of water and that is where my bafflles are placed so I have another 4" or height or about 10 gallons of extra empty capacity in my sump at all times so I never ever have to worry about a flood and can sleep soundly at night.
You would never want to run your sump completely full anyway so its the best most foolproof method. Check valves fail every day, it does not have to be a catastrophic failure, even a grain of sand, flake of food, algae, small snail, anemone, whatever can stick on the seat and even that trickle will flood given time. Same with drilled holes, you can clean the holes out this morning but who is to say your turbo snail doesn't park over the hole(s), a piece of algae breaks loose, your prized goby or other small fish doesn't get sucked up against it etc. Both require maintanance and there is no guarantee they will work when needed. An air gap due to exposing the returns cannot fail and requires no maintenance.
