They want a single-family home in a major world city in a walkable neighborhood, moving is not an option.

I hear this a lot - it’s a non starter. There are only a few places that are walkable in the US, and they are all expensive.

The suburban hellscape where you can walk to a church more easily than a bar is simply not a substitute, even if housing were free of charge.

Or try to get how zoning fixed to allow for mixed-use.