Excess water pressure can kill the embryos, but kingsnake eggs are quite tough in that regard as opposed to say python eggs. I have had kingsnake eggs with bulges from too much humidity when I first was learning to incubate eggs. Once corrected, they looked better and eventually hatched.
Your best bet is to have a closed plastic box with just one small air hole, and then a substrate like perlite where the water is below the top of the perlite. This way, the eggs are in a humid environment but are dry, not touching water. The air can be humid as long as the eggs are dry - no condensation on them and not touching wet substrate.Moss is wet, coco fiber is wet.
Some people use a piece of white plastic light diffuser - the sheets they sell at Home Depot, etc for diffusing fluorescent lights, the one that is 1/2 inch squares, over a substrate like vermiculite or perlite. This way the eggs stay dry for sure. The container can have just a single 1/16 to 1/8 inch hole for air. Some people use no air hole, just open the lid once in awhile. If you can keep the entire container at 80-82 deg, then condensation typically does not happen. It is when the container is exposed to heat and cool gradients that you get lots of condensation.
I use a styrene foam cooler, heat tape on the bottom, some plastic water bottles on top of the heat tape, and the egg container on top of the water bottles. I use a temperature controller to hold temps. No problems with wet eggs. I use perlite or perlite/vermiculite, the plastic grate, then eggs. Keep the cooler cover on. You can see examples on youtube. Way better than using moss.
