Pardon the bad quality, I just snapped a quick pic before I cleaned the brooder.
View attachment 2885261
These chicks are 2 weeks and 1-2 days old, my suspicion is that the real cause of death was mostly improper feed, and the heat loss sped things up. At 2 weeks they should basically be fully feathered other than the head. My suspicion is that although it’s always good to be diligent about bulbs and backups etc, once you get proper feed I bet your chicks will be strong and healthy and won’t be as accident prone. It’s a shame someone gave you bad info, but 2 weeks old with just primaries and all still alive until the heat went is really a tribute to the rest of your care being very good.