What happens to mains voltage in the event of a short circuit
If the short-circuit is an ideal short then the voltage drops to zero at the location of the short. If it's not an ideal short circuit then the voltage will drop but the actual voltage will depend on the resistance of the branch wiring and the resistance of the short itself.

What happens at other points along the branch will depend on the length the wires and their gauge. The voltage along the line will almost certainly decrease but determining how much the voltage will change is not a trivial problem and would require detailed information that you have not provided.