One thing I'm not sure of is how to frame the relative enthusiasm regarding outsiders with two alignments (Archons, Azatas, Demons, Devils) vs the singularly aligned (Agathian, Axiomatic/Innevitables, Daemons, proteans)
Are the singularly aligned ones "more" that alignment than a dually aligned one?
Are Agathians more good than archons, proteans more chaotic that demons, who are also less evil than daemons?
Has this ever been clarified, in either pathfinder or the older systems?
Not in any mechanical way (they react to detect alignment spell and smite alignment abilities the same way, they have equaly strong alignment auras etc.). However the creatures that follow pure concept may be able to effectively do more for that ideal that creatures that are bound by another goal. For example, devils try to create order, so they won't cause evil in a way that would destroy the order. Demons are too unorganized to be able to achieve some evil goal. Daemons on the other hand, to achieve their evil goals,use both law and chaos, as suitable, so they have more possibilities.
^This. Also depends upon whether you think of the alignment graph as a square/rectangle, circle/oval, or square/rectangle with rounded corners. I have seen both of the first 2 options (first is more common), and lack of the third may be due to limitations of the graphics available in 1st Edition (A)D&D.
Well, the singularly-aligned put more emphasis on that one part of the alignment spectrum than their dual-aligned brethren, but not necessarily in a way that makes them "better" at it. An inevitable's rigid focus on unbending law and order can get in the way of their goals just as much as it helps them, after all. And after all, archons and azatas both contribute to the cause of good in their own ways that agathions likely wouldn't be able to manage quite as well. That ability to cover for their shortcomings and play to their strengths is likely how the Good outsiders survive and thrive despite being so vastly outnumbered by Evil; a Devil, a Daemon and a Demon would never be able to work together without constantly being on the lookout for a knife in the back and a chance to put a knife in the other two backs. The worst Good gets is the Archon and the Azata getting on each other's nerves and having that sitcom "friends not getting along and trying to divide up the house" thing, while the Agathion plays the frazzled mediator friend.