i started megaton leeks last august from seed and planted them in the high tunnel over winter. they have all been harvested and sold over the last few weeks. i never row covered them and they did fine with the winter temps. I think this is too early to start them for wintering over.

We plant all our over wintered leeks at the same time we do onions and shallots. We have done king Sieg for years and last winter those things were fine with just a row cover in -17F conditions. We are finishing up harvesting the leeks and have a lot of nice big ones.

This year we are growing blue solaise which are supposed to be more winter hardy than the King Sieg

I always plant my leeks about now, and start harvesting them in the fall when they get large. When it starts getting cold I give them a deep fluffy mulch and continue to harvest until the garden is too deeply buried in snow to harvest. Once the bed re-emerges in the spring, I have just a very few weeks to harvest and eat the rest before they start bolting and get tough. The top gets frozen and looks relatively unappealing, but the part that is deeper in the mulch looks fine.

I am not a market gardener, but I think that having them in a tunnel to keep them from getting too cold will keep them looking more handsome for sales, and in really cold weather you may want to use spun bond row cover as well. IME leeks will continue to get larger, but not tougher, the longer the growing season, but will just stay the same size once the temperatures cool and the light levels decrease. I am not sure what the advantage of starting them later or the disadvantage of starting them earlier would be, but that may be because I have a relatively short growing season, usually about 4 or 4 1/2 months.

Mine were seeded the first week of April and I won't start harvesting till November.
I don't think it's too late to start some now, depending on the variety. I've never grown American Flag so I can't say much about them. I regularly plant the english hybrids (Belton, Kenton, Ashton.....) as they seem to take the cold without even noticing. This past winter they went down to 8F. for a few weeks without any issues. I think in your colder area Jay, the hoop house is a good idea.
-Mark