66F is fine for room temp...and it looks like they're inside the house so I doubt any serious 'drafts' are at play. They should be plenty warm going under the plate if they need it.....you would hear loud cheeping if they were too cold, did you hear that? The tote might get too warm overall, I would recommend against it. As to why those chicks died, it's very hard to tell...coccidosis is usually the first suspect in chicks that young. Did you see any bloody stool?.....tho that is not always present.