The mono valve is often one of the main problems in this car -- I have replace the insert a few times in the past 6/7 years.
You should not get heat from the center vents -- I wish that you could -- this would be great on cold nights with the top down. Only cold air from the center vents. When in "defrost" mode the center are close also.
I have had my climate control fixed many times -- the last time I also had the center vent blowing all the time -- It was the vac control for the defrost vents that was leaking -- it is inline someway with the center vent control.
I could not figure it out -- I have found that when it come to the climate control you need a dealer or someone who knows the exact model -- too many items can cause the same problem. Some are cheap to fix many are not!
Make sure that the inside temp sensor is clear of dust/dirt and that the air tube is connected and clear -- this will throw off the system if it is not right.