I think the first thing to do right now is stop feeding them. Bettas can live up to a month without being fed (not that it is ideal). But first, we want to rule out that they are not just bloated or the food is causing them to get sick. So lets fast for for about 1-2 weeks.
2nd thing to do: don't be so quick to dose too many meds at the same time. Be calm. Don't panic. If its a fast acting disease there is nothing you can do anyways. What you want to do right now is to minimize the deaths and hopefully save everyone.
3. dont take apart your 29 gallon. do a 30-40% water change. Take water out of the tank into a qt tank and put the sick bettas in the qt tanks. (do another 30-40% water change in the qt tank after a few hours, and everyday after until the fish is better)
4. Observe Each fish in the different qt tank separately. Diagnose their issue. Try your best not to treat all their illness together. Some diseases act faster than others. Treat for the most serious ones. (say if the fish has dropsy and ich and finrot, ich and finrot aren't quick killers, treat the dropsy first, then address the ich and finrot after.) Alot of the medicals have similar ingredients or conflicting ingredients, if you use two at the same time, you might be overdosing or over stressing the fish. Alot of time our fish dies not from the disease but the stress we put them thru while trying to save them. So don't panic.