So the AC was working OK until the monovalve was replaced? I agree, sounds like that's the culprit. Can you feel the heater hose coming off the monovalve into the heater core? If it's hot, but the system should be cooling, that would mean that the system is inappropriately sending hot coolant to the heater core when it's not supposed to. I suppose the problem could be upstream of the monovalve (e.g., a problem with the pushbutton control panel) but I'm out of my league here. Just a thought, though -- are you sure all the vacuum hoses got replaced when the monovalve was replaced? I understand that, for safety reasons, the default setting for the climate control is full heat, so I'd assume that if a vacuum hose (or wire, I don't know how it's controlled) is left off, it would default to full heat.
Hopefully someone with more knowledge than me will respond . . .