Wow..... I guess the only word would be..... YIKES!!! I guess you have a lot of work on your hands.
I have not heard anything bad about Magnolia Leaves. You might want to do a quick google search on them. Something to the tune of Magnolia leaves AND composting.
I would recommend going over them with the mower. And as long as I am talking about a mower; if your lawn mainly consists of weeds, so does mine
(which, I plan on ammending) and mowing weeds is just fine so long as they haven't gone to seed. In fact, Dandelions are great for the soil.
Good luck with your huge gardening project.
Actually, I would recommend planting Rye and Vetch or Buckwheat over the lawn first. The two grasses will choke out the weeds (a little shoveling before hand will help as well. Or just a rotatiller) Also, the Rye and Vetch are dynamic accumulators and after successive mowings (while leaving the clippings on the ground) will add nutrients to the soil that your eventual monocrop of a lawn will love.
And Rye (and I'm guessing Buckwheat) make nice lawns in the interm. I personally plant Rye and Clover in my Veg Garden as cover crops.
Oh yes, and the organic matter from the cover crops will increase the permeability of the your clay soil. If you can get a bunch of them from your neighbours (canadian spelling
) mulched up leaves will be great for breaking up the clay in your yard. Just apply them directly to the infected soil but be sure to add some manure, coffee grinds or grass clippings to the leaves as well or the soil organisms will take Nitrogen from the soil to break down the leaves. Best to give them a supply of Nitrogen from the "GREENS."