Hi there, sorry for the break. Here is my take on it:
1) I think you ended up with too short of an area that was actually bending with the added recurves
2) I believe the ridges did not contribute to it. To the contrary, I actually believe [no scientific observation] that the multiple ridges on summer harvested HHB if not violated might actually increase the surface area of the back. I believe it only becomes an issue if you have a heavily crowned stave. My HHB bow I made (Sudbury style) has extreme ridging and has so far not experienced any failures. It has however other than the ridges a flat back.
3) Backing will in my opinion transfer the problem to the bow's belly where you would increase the likelihood of chrysals if not careful. I had issues with that in the past, especially on shorter HHB bows (which this would be when counting the stiff recurves)
Cheers,
Phil