I know a lot about how municipalities work and there is no chance they regrade the alley for you.
I've also done a lot of concrete work in a previous life and you could set your floor lower and build a concrete curb on which to place the walls. If you do this be sure to pour the curb and the floor as one piece so water doesn't come in as would happen if you poured them separately. I've done this so feel free to message questions.
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