Hi Sheryl, Sorry about your bees, it's a tough loss this time of year. In the spring, the two most likely scenarios are starvation or queen failure, or mites because of what I say below.
First, did you notice any excess moisture inside the hive? That can chill the bees, especially during cold nights. The dead bees on the screened bottom board and lethargic behavior on a warm day suggest the colony was weakening gradually over time—NOT something that happened suddenly. This brings me to mite levels. Were there mites visible on the bottom board? If so, that could have been a major contributing factor.
Regarding the oxalic acid treatment in February—if the bees were in a tight cluster, the vapor or drizzle likely didn’t reach them well. By then, any viral or mite damage may have already taken its toll. I’d recommend considering a mite treatment earlier, like in December, when the weather allows for it. Additional, there would have been capped brood in February and in early December it would be doubtful if any at all.
If the queen failed or stopped laying late in winter, the colony would have dwindled without new bees to replace the old ones. Was there any brood at all? Only a few capped cells and no signs of spring buildup could point to queen failure or stress. Did you replace the queen last fall? How old was she?
Here’s my biggest question: was there any new brood? By now, you should have seen at least a full frame of capped brood and more uncapped. If not, how many bees were still alive, and how many frames were covered with bees? It could be that they just dwindle away
Lastly, I’m curious about the "heavy wax capping" over the honey. If you can post a photo, so everyone can see. Just make sure it’s under 4000KB to upload easily. Also, I’ve found that fondant tends to be too dry and hard for bees to pick up when it’s cold. Depending on their needs, they might not be able to consume much of it. If it was rock-hard, that likely wasn’t helping them much.