What do you mean by "slice it in"? Once you verticut and broadcast there's no need to go over it again.

I wouldn't let the seed just sit on top, I would go over it so that it was sliced in to the top layer

You can let it just sit on top, there's nothing wrong with that. It doesn't need to be buried. If you want to cover it so it doesn't wash away, or for water retention, you can use the thatch your verticutter pulls up and the clippings from scalping.

I've had much better results when I'm able to cover it and with that big of an area I'll just slice it in.

in terms of timing, is it better to have the seed down August 1 or sept 1 when its a little cooler?

I'm not familiar with temperatures in MA but I would advise seeding when daytime temps are on the decline but still hovering around 78-80. Heat helps a lot with germination. Maybe mid-August? You know best when that is in your area.