Martin–
To answer the question about legalities, yes they can do that. Is it illegal to delay renting a place, not necessarily– it is definetly unethical!
I would suggest doing the following:
1) File an apartment complaint against your property manager
http://www.rentalprotectionagency.com/html_apartment_landlord_complaint_help.html
2) In the complaint state that the property manager has been unethically slow with finding a tenant. (State the time-frames you mentioned above)
3) Resolution: Request that the agent investigate how the property was being marketed, and request that the property manager show how/ why tenant was responsible for rent payments during renovations.
4) I would suggest you add the option to have a certified letter delivered to the property manager from the RPA. (This is an option you can add to your complaint)
5) Take it one further step– State that management has had ample time to find a new tenant based on rent market conditions. Tenant is requesting that through Mediation the property manager agree to stop charging tenant for any future rents. Failure to do so may result in further legal action.
This should get your property manager in some hot water. I’m sure the Investigating Agent will hold your manager responsible to reasonable time-frames. If nothing is done, you will continue to have to pay your rent. Now go file an apartment complaint– don’t forget to add the “Certified Letter” option.

It doesn’t seem right that you would be charged during the time the unit is being re-finished. I would suggest filing a landlord complaint as mentioned above. I would bet that the manager will continue to charge your husband through the end of the contract unless you put your foot down.
Good Luck!