It sounds like you are a veteran weed-eater, too! :D
Some plants, like dandelions, have the best greens where they can grow fast and un-stressed. Where they are in the concrete, their greens are not so good. But, those scrawny, stressed plants can be a great source for flowers, as long as they aren't in a toxic location. I get some great dandelion flowers from plants that are in such bad shape! It sounds like your mallow is in the same situation.

I like borage, too. I think there's at least one salad that has it, in my post on making wild spring salads in 15 minutes or less. I'll try them cooked sometime, too. Thanks!