The zinnias I planted in a raised planter on my patio developed serious powdery mildew. After discarding the plants for the fall, can I keep the soil to reuse next summer? And if so, can I just cover the planter for the winter? Or should you always discard used soil every fall anyway?

You can cook the soil in your oven but it smells. I forget the temperature because I don't bother. I use 'used' potting soil (uncooked) for transplanting tomatoes to larger pots and for new flower pots. It seems to work OK.

I would not with that powdery mildew issue.

You'd be OK if you're not planning on growing zinnias or anything else susceptible to that particular powdery mildew, which unfortunately has a wider host plant range than many. Some zinnias are more resistant to powdery mildew, I've never seen it on the Profusion ones for example.

My used potting soil generally sits in buckets of aridity for a year, then gets added to healthy transplants the next year.

Plus, since it exists on lawn grass and is blown around by wind, its pretty hard to avoid.

There are different species of powdery mildew and many are quite specific to certain plants although unfortunately the zinnia one (Erysiphe chichoracearum, which I think has had a name change but many references still use the former) can affect many other plants. The powdery mildew on grass is a different fungus, Erysiphe graminis.