How to deal with my thesis chair?
The thing that stands out here is that the new advisor is going on medical leave and won't be back. This sounds serious. Maybe he is facing his impending death. This would make his work very unpredictable. 

It sounds like the department head was trying to make the best possible decision for you, though probably shouldn't have done so unilaterally. But starting over again will probably just delay your completion, which I assume you don't want. 

You've been caught in two bad situations which is very unfortunate. Work with the head to find a good way to reach completion and also get good recommendation letters to support your future. 

At least ask if the head can find a way to give some stipend to your first advisor to permit her to continue to work with you. Even if that isn't possible it may get the department head to start thinking of better solutions. It is a situation that is uncomfortable for the head as well.