15 degrees is probably a bit too far advanced! I'd go for around 8-10 degrees dailying, 12 max.
Still some air in system? How do you know the head gasket is not leaking? It's pretty hard to have an air pocket in a k motor after changing the coolant, I'm not sure I could do it even if i tried. After everything you have described this is telling me that there are combustion gases getting into the cooling system from a dodgy head gasket.