I use these attached to 4x4 post used for pens. One on each side of post, one with grit one with OC. I just drilled holes in bottom to drain, have had them for 10 years now work great for me.
I would not make a safety hole until they have internally pipped. Making a hole too soon aids in membrane drying out. Being patient is your best option at this point.
:welcome Sorry to hear all your troubles if that is your egg in pic that is on day 24, it is very underdeveloped and doesn't look right from picture and not likely it will be a successful hatch. If all your eggs look like this at day 24 of incubation it doesn't look good for any hatching.
:welcome Welcome 1st place to start would be your actual temps and humidity in incubator, not just trusting your incubator. As they are usually wrong, and nothing runs perfect forever then we might be able to go from there. Sounds like your machine may not be holding correct temp as it used...
Yes, that maybe it the air cell is very small, but I would check temps throughout your incubator. As the egg in pic was way underdeveloped for day 18 sure signs of low incubating temps also. Incubators tend to have hot and cold spots, most will recommend additional calibrated thermometer and...
From that pic it does look like a blood ring but at only around 7 days you may want to check again in a day or two. But I do think you spotted a quitter.