I still see no reason to look forward with anticipation to the first killing frost. There are years with no warmth following that freeze. Last year in Wisconsin, for example, we had a killing frost in September with October reaching temperatures well below freezing and frost fingering its way into the ground. November could have been considered warm next to October, with its normally cool temperatures, but nothing like we associate with Indian Summer.
Your mums will be happy with the gradual decline in average temperatures that comes with September. From an average daytime temperature of 81 in August, we fall to an average of 62 in October. Certainly more comfortable as winter approaches. No need to wish our beautiful autumn period away in hopes of an iffy, fleeting chance at a week of Indian Summer sometime in the future.