Your reasoning is sound, but it seems a little odd to appear to go from one site design to another whilst moving pages. Have you considered linking to the subs within subs the same way as you link to them in the main page (that is, in your example, P2 would end with two links to the other sub-pages). That puts them in a natural reading position (you've learned more about one thing, now learn more about something else), and it means it doesn't 'feel' as though we're suddenly being dropped into the middle of a brand new navigation system.
I certainly wouldn't add any other layers to the header navigation bar, though. It would be hard to maintain visual balance, and it could easily get messy for large bodies of subpages.
The only other method I can think of would be a rather flashy (and potentially tricky to develop) set of animated transitions. The user would click a navigation image, and the body content above would 'compress' (contract to leaving a header), with the navigation images moving to the top and body content for each subpage appearing below. Your users would visit other subpages by clicking the other images, or return to the 'parent' page by clicking the header. Then, your three images for each page would look like a natural extension of the horizontal nav bar in the header. It's a thought, anyway.
