50% every other day isn't "overkill". Its just ensuring that the water is gonna be nice and clean. Even with discus you don't "HAVE" to do it. Only reason we do daily changes with discus is to get them to grow fast, and keep the water super clean which helps them grow fast. You can keep discus in a tank that only gets weeklys, but its not gonna have as nice of a shape or be as large as an adult that was raised with frequent changes. Doing a change 2-3 times a week isn't that much work, takes me 10mins to do one of my 55 gallons each day, and thats with wiping the tank walls down, and cleaning filter pads. When it comes down to it, the more fresh water you give fish, the healthier they are gonna be. Whatever happend to the fish in this instance wasn't water quality related. Lack of O2 in a tank will have the fish gasping at the surface, so it wasn't that. Sudden temp change? Please, many fish require rapid temp changes to induce spawning, a 5-8 degrees isn't uncommon to induce this. Discus can handle it and more, as well as many many others. So unless the temp drop was 12-20 degrees, its not gonna bother them too much. The kH in the tank is high, means super stable water going into it, that translates into a near impossibility to have a rapid pH change, even in that case, most of the time they hang at the surface gasping for air, not lying on the bottom.
The baking soda won't hurt anything, just add it anyway to be sure. As to the epsom salts, a quick increase in gH won't bother the fish. I'd toss in 3-4 handfulls of epsom for that tank. Discus fry go through it all the time, soft acidic water with their parents, then into a hard alkaline environment for grow out with no adjustment. With the speed this hit those fish and what it was, some chemical was added to the water. Some of them faired better than the others.
If your water bill is high, your doin something wrong. I'm on city water here, have 11 tanks running, half of which get daily 50-75% changes. And the total cost for all of them is about $70 a month, that included electrical and water usage (Probably lower considering I just switched some over to sponge filters and insultated tanks so the heaters came on less)