Lime
D/L Neutral means that it will flower whatever the day length. That's why we growers love them so much. It's really your decision whether you sow it first or last although if you want flowers for fall, I'd be inclined to plant it later in your zone.
With regards to your 2nd question, IMHO it's longer, they like intense light, so the more hours of light you give em the better. I haven't been doing this as long as some of the other posters on the forum, so it'd be good, if some of them would throw in their two cents worth.
Cheers - Kim
Billabong Fresh Flower Farm

According to research at the Kansas State University, most sunflower cultivars appear to be day-neutral. However, their research has found, and we conclude as well that sunflowers tend to flower faster during short days -- a difference sometimes of 7 to 21 days. We have noted during the short days at the beginning and end of the growing season that sunflowers tend to also bloom on shorter stems with smaller bloom size as well. These findings are true regardless of whether the sunflowers are grown in our greenhouse/hightunnels or in the open field.

Succession sowing every two weeks throughout the season will provide blooms for your garden enjoyment. It's always a bit trickier for us flower farmers who grow our sunflowers for market. There are so many variables, and, there are times that we find ourselves harvesting those beautiful blooms from not one, not two, but three sections. This means that we need to perhaps find more outlets for our sunflowers during this one or two week time frame. It also means that there most likely is going to be a week in which our flower farm has no sunflowers for market. Our market customers seem for the most part to understand this.

On our flower farm, we have found certain sunflower cultivars work best for us here in the Upper Midwest at the beginning and end of the season under both greenhouse and field conditions. And, we grow other types during the long days of summer.

Thanks for the insight -- I didn't know any of this!

My bright idea is to grow little shortie sunflowers in pots. Maybe 5 or 6 seeds to a pot.

Lime