Sideslip does build during a spiral dive though? The spiral tendency being partly a balance between directional stability reducing sideslip and the dihedral trying to use said sideslip to roll more wings level? So an aircraft with more directional stability will have a greater spiral divergence tendency (all others equal), and an aircraft with more dihedral less of a spiral divergence tendency (all others equal)? Worst being an aircraft with strong directional stability (big fin) and lesser dihedral?