Re: cold water missing
It is a common problem with older plumbing, which is why pressure balanced shower valves have become a code requirement. When the toilet is flushed, the cold water pressure drops significantly, causing more hot water to flow than cold, resulting in the scalding. A pressure balance valve senses this pressure differential and prevents a scald situation from happening. Whether there is something other than replacing the valve that you can do, I do not know.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!