I believe you are correct about calendulas being a cool-season annual. In my zone, heat runs through October. Your fall probably sets in earlier, but I still think you are a bit too early to start seeds. Just my 2 cents. I hope someone in your zone replies.

Calendula is indeed a cool season plant. Planting them in your fall is a good idea but I wouldn't start them uintil the weather cools a bit. Calendula germinates at a temperature around 70 degrees. Too hot and you'll kill the seed

Funny that you mention this, as just today I cut back all my calendula by about half. They look much better and I'm hoping they will continue blooming. Maybe heat will do them in anyway?

^^ I deadheaded and cut back a bit, too and they went downhill and looked like utter cr*p after that - they were in a location with hot, all-day sun, so maybe if yours are in a less harsh exposure they'll do okay, who knows (but keep us posted).

My calendula self-seeded this spring from last year's plants and have just begun to flower. The ones I started from seed indoors in April have been flowering for 2 weeks. Take it from there.

There are many varieties of calendula depending on the seed source, in my experience. I grow it every year. Each would perform a bit differently. I deadhead my plants religiously, so that may affect a good performance.

Any experiment with seeds is worth a try in my book. Take notes and learn from them.