Micro bubble/Sump overflow issues

Drilled holes and check valves are both a false sense of security and will fail period.
The ONLY proven, foolproof method of backflow prevention is an air gap. Simple, zero maintanance and it cannot fail, ever. We all know wate rcannot jump uphill so when the return is exposed to atmosphere the siphon quits and flow stops. It does not get any simpler, does not require cleaning or adjusting and is idiot proof.

You can have a drilled hole or holes but they get dirty and need cleaning and even then whats to say a snail doesn't park over the hole, a piece of algae gets sucked up in it when the power goes off, you anemone, a small fish, food, anything. Same with a check valve, all it takes is a grain of sand on the valve seat, or some slime we all have in our marine tanks. It does not have to be a catastrophic failure, even a trickle will flood in a short time when you are away or asleep.

Keep the return(s) close to the surface and maintain freeboard or extra room in your sump and you can sleep soundly at night without worrying about a flood! Its very easy to calculate how much extra room you need in the sump. Take your tank length in inches, times the width front to back, times an inch or 3/4" or whatever level it will driop in a power outage then divide that number by 231 and it will give you gallons.
In my case I have a 60" long 100G reef that is 18" front to back and my returns are 3/4" below the surface. 60x18x0.75/231= 3.5 gallons. That is a maximum of 3.5 gallons that will flow to my sump in a power outage, not much at all really and since my sump is a 30G and I know my skimmer works best with about 9" of water the sump has at least 8-9 gallons or more of extra capacity in it at all times so a flood is not possible.

Forget the drilled holes and check valves, the cleaning and maintenance they require that may or may not do anything at all and simple keep the returns close to the surface and provide some room in the sump and you are much better off without spending a dime or doing any work.

On your return, install a piece of Loc Line extending up closer to the surface or add two back to back 90 or 45 degree elbows to raise it up closer to the surface. Your other option would be to shorten the two "over" baffles in your sump to lower the level in the sump which might be a good thing anyway to help keep more water in your return section which appears verl low, almost to the point of coning, vortexing or cavitation of the return pump.