We carefully piled up stones inside the arch opening until we reached the peak. (This was the scariest part of the operation)
With all the stones in the arch now supported, we removed three or four of the slumping voussoirs. This allowed us to add some more height to our temporary stone form with smaller stones to give us the gothic shape again. After that we rebuilt the arch with several of the original flat voussoir stones, as well as a new beefy keystone.
Then we carefully removed the stones inside the arch.
Voila. Open arch surgery was a success !