What is it you expect this "topsoil" to be? The definition of "topsoil" is that it is the top 5 to 6 inches of soil from someplace so unless you specify what you want you could get anything and most of that could be not what you want.
I would look for some good quality compost to work into that clay, but look at what you are proposing to buy first to be sure it is what you want.

By topsoil I mean something like a mixture of a black loam, some sand, and compost. The more I read, the more I'm thinking about going with mostly compost to increase the organic materials available, then perhaps do a compost tea treatment.

That is what I thought. Loam is a specific soil type and is about 45 percent sand, 25 percent silt, 25 percent clay, and about 5 percent organic matter. If you can find something like that it might be worthwhile but if you can find some good compost use that instead, keeping in mind that you will need to add some compost every year.

I've ordered 50 yards of organic compost from Living Earth to put on top of my clay yard. Keep in mind that this clay (about 12k sq ft of it) was recently spread and originated well below my surrounding topsoil layer so has no real life in it...came from holes from a pool and an aerobic septic tank. Will spread about 1-2 inches of compost on top of the clay. May use a bit as a top dressing on the remainder of they year (mostly weeds) to amend that part of the yard, and will use some in new flower beds. Will level compost next week. I would not typically till the compost in, but, given the depth of the clay in the yard, should I till the compost into the top 2-3 inches of the clay? If I don't till, can I seed and sod on to of the compost?

Tilling the compost in will be a better option then seeding into the compost laying on top if the clay, mostly because the compost may well hold too much moisture since it would not drain very well into the clay.

Wow, I can't believe you kept your dug up clay. I simply couldn't put even the smallest amount of clay on my existing front yard grass, it just lays there blocking the grass. Grass is growing on my existing clay front yard and was on my back, but I removed enough clay from the back yard to slope it for drainage. I then covered with topsoil and sod (8 yards topsoil and 185 yards of sod).

Have you checked local college/university agricultural extension offices (web pages). They have excellent detailed information for local soil management issues and recommended plants.