As regards Succubi, I think their presence in the Abyss can be explained quite easily. We know that Ice Devils are actually captive demons. So, if Succubi are actually devils, what do they do in Abyss? When the question is so stated, the answer is obvious. The usual duty of Succubi is to seduce, corrupt and control. So, they have been sent to Abyss in order to control the demons.
It also pretty well explains the reason why both Malcanthet and Shami-Amourae oppose Demogorgon in the Savage Tide Adventure Path - that is the reason for which they have been sent to Abyss. They were to control him, and when that proved impossible, to betray and defeat him.
Similarly, I think the new system fits Greyhawk much better than the old one. In the Great Wheel cosmology, the true war between demons and devils took part on the planes; the Greyhawk was merely a backwater, with all the strategic significance of Africa during the First World War.
Now, the Material Plane lies between Abyss and Astral Sea. The demons wanting to go upward will naturally pass by it. On the other hand devils must be summoned by someone to appear outside of Hell. And that someone will usually live on the Prime plane, not in the Astral Sea.
It seems clear that Grazzt fathered Iuz to destroy Greyhawk - you cannot destroy a whole world with your bare hands, a bit of ingenuity is called for. Iuz himself probably wanted only to rule, but had to rely on his demonic allies - and as the result his Greyhawk wars had inflicted a mortal wound on the old Greyhawk.
The Great Kingdom and Scarlet Brotherhood summoned devils as allies against demons, and the devils corrupted them (eg Ivid the Undying). The rulers are probably all possessed.