I don't see any issue with air flow through the purlins and R panels. What I haven't seen is any soffit vents and ridge vents to pulle the air from the bottom to the top. Air flow is all you are after, but there has to be a start and and end to get it going. Having smaller openings just creats a venturi effect and should speed up the air going through those areas. In theory, it might even help by having bigger and smaller openings. Either way, I don't see a problem with the way the roof is built. I just don't see how air is getting in and out of the space?
Again... Be very careful of over thinking, over engineering and over doing a fix. The easiest isn't always the best, but the most complicated and expensive never is the answer either. Keep it simple and stick with what is proven to work. Either create proper air flow or seal the space.
There are lots of types of foam out there and the goal isn't to get an air tight seal, just to eliminate the vast majority of the air space so there is less volume of air in the cavity, resulting in less moisture that the air can create condensation.
Eddie