Regarding Concept Assessment itself, had it reached more precise conclusions, it would have hopefully dictated a better outcome in all the later stages. However, I can't agree with Deck that it's solely to be blamed here - at least I came out of that stage with some conviction that 'we knew what we were going to do' (misguided as that was, heh).
The stage where things went wrong I'd say was Abilities. Weak Armor was great, but mixing in Illusion and No Guard to outclass it ruined CAP4. And then things went downhill from there - if not for the Abilities chosen, our Stats and Movepoll would've looked differently, or at least I hope they would have.
And still, if Aurumoth had two other abilities (or even if Weak Armor was its sole ability), it'd be riskier with its current stats and movepoll than it is now, with what Illusion and No Guard enable. So if I were to go back and redo just one stage, that'd be my choice.
In short, the lesson to take away from this (besides 'pin down vague concepts') would be "don't overshadow your primary ability with stronger secondary and terciary abilities". A lesson we needed to repeat, apparently. Hopefully for the last time now.
I'm a casual Smogoner that mostly frequents the forums - chances are you won't find me on IRC that often. If you want to talk shoot me a PM please, I'm not on every day but I'll eventually reply.