I think we're kind of getting off-topic again, but I'll try to address the "contradiction" briefly.
The idea that outsiders, in some way, represent an ideal is true a certain extent... planar lore across several editions has talked about the Astral Plane as a place where thoughts become reality and ideas are as powerful as physical forces. This is generally consistent with Planescape's metaphysical approach that belief shapes reality. As a result, outsiders are, to a certain extent, embodiments of abstract concepts as well as physical beings.
But I think what some people are missing is - again - a conflation of "ideal" with "perfect." That's not really what the lore means in my opinion.
Devils are an "ideal" form of lawful evil (and evil lawfulness); they are the representation of everything oppressive about law and everything tyrannical about evil. They are not a perfect representation of complete, unadulterated evil or of complete and total law. A devil's lawfulness actually limits their capacity for complete evil and their tendency towards evil limits the degree to which they can be lawful. The same is true of a demon in respect to its relationship to chaos and evil or an archon in respect to its relation to law and good.
Devils and demons are both "ideal" forms of evil, but only in a specific respect (tyranny and wanton destruction respectively). If you're looking for "perfect" embodiments of evil neither is one, because the nature of their evil is colored in a particular way. As far as perfect forms of evil go, yugoloths/daemons are closer, as they represent pure, unadulterated selfishness (or avarice, as the 5e MM describes it). But again, part of the purpose of the distinction between LE, NE, and CE is that which is worse lies in the eye of the beholder.