Couple thoughts.
Does the car run? If it does turn the heater on and see which hose gets hot first. That will be the supply
. Don't forget to let it cool down before you start disassembly. I would just flush both ways through the heater core if I was doing it. The passage ways are pretty open on it.
In regards to the monovalve if you have a breaker yard nearby try to grab the monovalve from any of the early 90's MB's that had them. They won't be like the 95 E300 but you can mix and match parts as needed. The main thing is to not let the body of the monovalve get messed up. It may be a single year design
- not sure. Not sure which all monovalve bodies are just alike but there are similarities between them.