Suggestion: set up a test harness model first.
Use a reservoir, at high pressure and some subcooling, as inlet to the valve; use another reservoir, at low pressure and some superheat, as outlet of the evaporator. Test your valve and algorithm in that model first.
Then make the inlet reservoir a controlled reservoir, vary pressure, maybe also subcooling, as a function of time, see if your expansion valve is able to maintain a superheat.
A smaller model may also help the community look at your model, if you'd like to share.
