Just for fun, search this forum for "harvesting onions".

You'll find pictures of Don's amazing display of onions.

I'm sure he'll reply to this post soon, and tell you all you need to know and more :)

Seems I missed this post earlier...

If when you say "sets", you mean small bulbs that can either be grown for "green onions" in 4-6 weeks, or as "bulb onions" if you leave them to mature until the tops die in late summer... in my experience those will ripen much earlier for you than onions started as seed indoors in February and hardened off and transplanted to the garden in May as actively growing plants. But the "sets" or bulbs will also produce much smaller onions than onions grown from seed started in February or early March.

As for the onion maggot treatment, I don't think I can be of much help there as I use chemical treatments, not organic. Usually around mid to late June, I make up a solution of something like malathion and used that as a soil drench to kill any onion maggots. Since the directions say don't harvest for 2 weeks after treatment, whereas I won't actually harvest for 10 or so weeks after treatment, I don't worry about using a chemical treatment.