I think your first proposal addresses the root cause.

But the second proposal - forcing municipalities to write options with unlimited potential losses - is silly. The whole point of the zoning authority is to say "it'll be best for the neighborhood in the long run for there to be some retail shops here", and it makes sense whether you're talking about a lot or the first floor of a building. If the development isn't profitable with the ground-floor retail requirement, then just don't build it.

Some sort of option might work, but it would have to be something like: the municipality can choose to rent the space for a fixed price if it's vacant for long enough. This encourages the landlord to rent it for some higher price.