McChooky
Free Ranging
Newly hatched chicks don't have enough body fat to help regulate their body temps. Once they get hot it takes too long to cool off .Chicks have to have a warm place that doesn't get too hot otherwise they will move out from under the light and stay cold. Unstable body temps of the chicks kill them like this.If you look at their feet it should show they are dehydrated if this is the case.Some, not all. They are fine and just fall over dead. Most things I can think of they should be lethargic before they die. It is spaced out over a few days, not all of them all at once. It's possible one or two could have a birth defect so they just fall over but not in those numbers. That seems warm but they should be laying around panting if they are too hot. Still, I agree they need to be able to get to a cooler spot. That's one of the problems with using a bin, they can hold in the heat and not provide a cooler spot if you are not careful. If they are not at the edges this isn't your problem.
My first thought was air quality. They have pretty delicate respiratory systems. If they were near an open flame like a gas water heater possibly the exhausts were settling in the brooder. The way you describe the brooder that doesn't sound likely or they would all be dead. A photo of the brooder might help. It does have decent ventilation?
I remember one similar story where the people actually got back with us when they found out what the problem was. Their kids had been feeding the chicks some old gerbil or guinea pig food as treats that had been poisoned while in storage. When you first get them they don't all start eating and drinking immediately. The first to eat or drink could fall victim if this were the problem. But it's been long enough that they all should have been eating and drinking for a while.
I assume you had your well water tested before you started drinking it? Softening it should not poison it. You aren't carrying it in a bucket that once held poison are you? I'd be more concerned over something like that than water that is safe for you to drink. I don't see anything wrong with switching to another water source though. It is possible there is something there.
I can't think of anything that fits those symptoms. It's not all of them at the same time (or reasonably close) and they aren't showing any symptoms before they die. It does not sound like anything from the hatchery or during shipping. I think it's something that is going on where you are.