Your lack of heat, and the issue of the car stalling, are likely unrelated.
Your new monovalve could have been defective from the factory. New ones are not made like they used to be.
Another potential issue is that the electromagnet in the monovalve is going bad.
In either case that would lead to a failure for the monovalve to close properly allowing hot coolant to bypass the heater core.
The stalling issue is likely something else entirely and likely related to your electronic fuel injection. I unfortunately do not know about those systems. Hopefully you'll find an answer for that somewhere else!
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