If you have access to a hand vacuum pump it would be handy to check the vacuum doors. I have seen the diaphragms go bad before. Another thought is that if there is a vacuum leak most systems default to the windshield defrost position for safety.
Yes it does sound like the valve is not the problem. In my case the valve would close off completely when it was cold, but as soon as the coolant warmed up some it would close but stay slightly open. It was enough to allow hot water to flow through and give hot air flow.
Hope when you find your problem you will let us know what you found.
