I don't think the mantis' will drown if you don't get real sloppy with the water, but I had some crickets just walk right into a puddle, very small puddle of a couple drops of water that was condensed on the side of the bag they were in, that ran down the bag and formed a large drop. Stupid crickets walked in and drowned, in one big drop of water... I don't think the fruitflies do that, but I don't know.
I had one mantis climb on my hand when I wasn't looking at feeding. When I got done I went in to wash my hands and the mantis was in the bottom of the sink looking drowned and like a limp rag doll. I took him into the area where I keep them and put him on a towel to dry, he looked dead and I thought he was, when I came back later he was sitting there upright like nothing happened, so they can take a good wetting, as long as they get out of it and dry, sometimes.
I'm new to this also, but I usually mist two or three squirts into the jars I have, maybe only one if I have stupid crickets in there, which is rare. I then moisten the top when I get down. People here have said they think that'll cut off air, but as dry as it is here I think it dries so fast that they would never even know the difference.
I do think though, that if I had a plug in the side that air could come through, as well as through the top, and then I'd keep the top moist more when it appears close to molting, so molts go easy on the guys.
I don't use the deli container for my mantis', except one. I may go to them though for ease all the way around.
I put some air plants in a couple containers this morning, and I'll see how they like them. So far the one seems to like it, and I think the fruitflies hide in between the leaves or fronds or whatever they are.