Not all that uncommon to have the semi-doubles showing up in zinnias, regardless of the color. Completely full doubles are a little harder to produce genetically. Cultural conditions also have some effect. Stressed plants often seem to be less successful in flower production.

There aren't all that many greens among the zinnias, so I suspect they aren't as strong overall. Tried a new one this year and found it smaller than I'd hoped. We did, however, have one of the driest seasons and the plants got little supplemental moisture, poor things. BUT, this is probably the first year I haven't seen mildew on zinnias. Amazing.

Mildew is pretty common a problem. A few varieties are a little less susceptible, but under the right conditions it will pop up. Keep plants spaced apart for good air circulation is the first step in reducing/slowing mildew.

I also planted the green zinnia, "Envy" and only had one flower that came true out of the whole package. I was not impressed. The rest were as you described. I also had a dry year, but my other zinnias did much better. If all the greens had turned out as nice as the one flower, with many petals, and definitely a true green color, plus it lasted about 3 weeks, I would have loved them.
Cheryl

I grew 'Envy' for several years, had some of the same problems you had. This year, I grew Benary's 'Giant Lime', and it was much better. Lots more dbls., taller stronger more productive plants, despite a horribly dry season, w/ record high temps. The color is not as pretty when it's very hot-more of a sickly yellow, but the cooler weather is bringing the green color back.

I've grown envy before and this year tried the Benary's Lime...and I don't think it did any better than Envy. But, my zinnias are overall not doing as well as in the past EXCEPT for the mini's Persian Carpet and Old Mexico...they're doing great - sell out of them every week!
I'll give the Benary's Lime one more year...different growing conditions, etc. and see. But, overall the green just aren't as great...except for the occasional one!
Wendy