I have a potential garden plot that has years of swimming pool DE back wash in the soil. The possibility of chemicals in the DE is a concern. Is this OK for a vegetable garden?

As you may know swimming pool DE and agricultural DE are two totally different products.

The chemical DE in the soil will not stop your garden plans, if you use tons of organic matter in your gardening system from now on. The soil microbes will recover quickly. I believe I read one expert say from about 6-12 months max.

However, in the short run, you can use lots of dry molasses powder and/or aerated compost teas with molasses in it, as a soil drench, to revive the biology in your soil.