Hi Penny, it looks like your whole block needs serious work done on the hot water supply. Obviously it can not cope, and as you say, the very hot water is dangerous.
Old houses have problems when a tap is turned on if someone is in the shower. Sometimes you are standing under cold water, or very hot water, or have a tremendouse noise with pipes banging.
The Plumber who installed our last hot water service would not set the temperature below 67 degrees, because he said, bacteria could grow? Also the lower the temperature of the water, the fewer hot showers/baths you will get out of it before the water runs cold.
You definitely should be able to have some sort of tempering device so that the water is not able to fluctuate, if the current system is able to cope.
Hopefully some Flatchatter plumbers will have some good advice for you.
Take care.