Why crab apples? They’re small and don’t taste very good unless cooked. It’s food security only in the very loosest sense of the word. Asian pears are easy to grow and very tasty. Wash 'em off and you’re good to go.

Apple trees need to grow for years and be properly trimmed so they make the apples we eat. Crab apple trees dont make good apples at all. Also as you said the soil may be contaminated which is something to be aware of if you’re attempting urban foraging.

However thats not to say you shouldn’t plant them. Just don’t expect to feed anything other than the birds. More trees are always a good idea but I would stick with what is native to your area. Also be aware when you’re planting in empty lots that they could very well be chopped down when it is developed. Try to plant around the edges of the property in spots that would not be in the way if something does ever get built there.

My city actually has a program where they incentives new tree planting in urban areas. I think they will give you a good deal on land bank properties if you promise to keep a decent green space going.

Depends on the area, it’s always better to plant something native to your area. If crab apples are native to where you are planting, go for it.

Why crab apples? I have a crab apple tree and make jelly every year, but I can’t give the stuff away. Lots of tannins.

Lead is less of a risk for tree fruits. I agree about wanting to find native options. Talk to a local arborist. In my neighborhood, we planted pawpaw, golden currant, and some sort of native plum. This is central US. One thing that you definitely want to avoid though is monoculture. If you get a bunch of the same tree species from the same place they can be clones of each other and be at very high risk of disease.