This is all happening too fast for me. Two glasses of the Coonawara shiraz and the debate has gone out of control behind my back.
I am now happy in my own world that longitudinal dihedral does not directly affect stability. However, I think I have convinced myself that an aircraft in level flight cannot hold a trimmed attitude without a degree of longitudinal dihedral.
That is why models, with fixed control surfaces, have it.
In BEagles book, the rise in restoring tail moment will be larger than the unstable wing moment so long as b x St is greater than a x Sw ( for equal increases in alpha).
Dick W