This looks like multiple situations condensed into one. I will share my thoughts on the lease breach.
a) walk-through report. I would suggest to go over the walk through to review the condition of property. Technically tenant is responsible for damages beyond normal wear and tear, there maybe some items still under their responsibility that you would consider to have them responsible for.
b) Per lease agreement, the moment they breach the lease, the deposit goes to you, you are free to do anything you want.
c) & d) Communication & Remedy: Personally I think this would depend on the situation. If through communication can reduce the bitter feeling and avoid taking this case to court, I think it is the best interest of all parties. If you have everything well documented and work along the regulation, I don't see you need to fear even if they do decide to sue you. But again, if this can be avoided via communication, that would be the best.
Hope my input helps in some way. Again, double and triple check local regulation to make sure you are following the rules.