Question
I've enjoyed reading through your many answers to several situations similar to mine, but I'm going to be selfish, if you don't mind) and ask for a customized answer to my yard.

  We moved into our present home near London, in southwestern ON, just over a year ago.  The previous owners had dogs, and the 3500-square-foot yard was full of clay, urine patches, very little topsoil, and most of the yard is in full sun from 9 am to 7 pm.  Where there is some shade (along fences and on one narrow patch on north side of house), grass flourishes quite well.
  My first move, last year, was to aerate and put down peat moss - lots of it.  My thinking was to increase air and water flow.  I also topdressed with ~50 25L bags of "black earth topsoil" from a store, seeded with a sun/shade mix, and fertilized 2X.  I think I spaced these out properly throughout the year.
  This year, I added another 30 or so 25L bags of the topsoil again.  This amount hardly puts a dent in the landscape, but I figured at least it was something.  I also aerated in the spring.
  The results have been much the same as last year.  Tons of crabgrass growing everywhere, and huge brown patches where sun is prevalent.  I'm hoping that a lot of the grass comes back in the fall, but I'm not terribly hopeful.
  Reading previous answers about the sterility of peat moss, I think I need to introduce a lot more "life" into the lawn.  If I take out a plug of soil, there is what appears to be about a half inch of thatch (I mulch and leave grass clippings), an inch of peat moss, then clay.  I don't know where the topsoil went!  A couple of times, I've tried to rake up what appears to be thatch, but I don't want to be too vigorous and rake up the peat moss as well.

Please check the following plan I've come up with and advise on it's correctness:

1.  Put down an inch or so of bulk compost ASAP(is horse compost or mushroom compost preferable? Those are the options at our local garden centre.)

2.  Try and find earthworms sold in bulk and introduce those to the lawn/compost ASAP.  Also, I've heard many good things about beneficial nematodes.  Should I introduce these as well?

3.  Buy a truckload of two-way soil (topsoil and compost) and spread THAT over the lawn.

4.  Aerate in mid-September.

5.  Spread sun-only-mix grass seed for overseeding purposes.

6.  Fertilize in mid-October, ~ 6 weeks after overseeding.

7.  In the springtime, put down cornmeal gluten to control weeds and provide slow-release nitrogen.

8.  Aerate.

This is a lot of info, I know, and I appreciate any input you might have.  And I'm not too proud, so be brutal in your assessment of my actions past and planned!!
Thanks!  

Answer
It breaks my heart to see good people sweat so hard and not see any fruits for their labors.

Let's go down your list and see what we can do.

1.  Horse Compost - this is Horse Manure, Composted?  Fresh Manure contains Weed Seeds.  You need those like a hole in the head.  Mushroom Compost is a good choice too.  ANY Organic Matter you incorporated into your Soil is good for it.  You probably have Earthworms, but if something caused them to vanish -- heavy tilling for instance or a lot of chemicals (which it sounds like you have avoided), Earthworms would run for the hills.  They breed better than Rabbits, though.  Check to see if you really need them.  If you have them, don't waste your money on more.

Beneficial Nematodes live in the Soil naturally unless they are eliminated by the Forces of Man.  Mail Order Nematodes are highly delicate and easily killed by something as ordinary as Sunlight.  Grow your local Nematodes population by not using chemicals and keeping your Soil healthy and Oxygen-rich.  Nematodes cannot survive in compacted, unhealthy soil.  Plus they cost an arm and a leg.

3.  For future reference, 'top soil' can be anything the cat dragged in.  It can be filled with Weeds even if labelled 'Weed Free'.  Compost, Humus, these are terrific.  Your Peat Moss was a good try and probably did wonders for soil tilth.

4.  What kind of aerating are we planning on doing?

5.  What kind of Grass Mix are we overseeding with?  Probably not important but I would like to make sure.

6.  Fertilizing sounds on target as far as the schedule.

7.  Cornmeal Gluten is tricky, like all pre-emergents.  Watch to make sure it does not go down too late, or too early.

8.  More on aeration, please.

Have you done a soil test, my friend?  I don't see that on your How I Spent My Summer Vacation.  All that work should include a soil test.   It will guide a lot of your decisions.  rsvp