A defeat is still a defeat, this is true: yet we have to take a few things into consideration. Manchester City after the re-organization (which I believe came to late) were the better team. This cannot solely be attributed to the opposition taking their foot of the gas. This was a derby; the mercy rule does not exist. United had the chance to avenge the 1-6. The notion that they eased up is preposterous, cautious once City made it 1-3 maybe.
The tactical change was a beauty. Clog the wide areas to shut out their first half threat. Keep a tight block of two holding midfielders, thus allowing Aguero and Nasri to still support each other with the occasional break from deep. All this time City's disciplined shape never faltered. This was the key, City never onced chased the game, for if we had, United would have exposed it. Playing with ten men is an art and you need to always have two players left up front to feed off each other once the ball is finally played up there. The alternative is nine men behind the ball and relentless pressure, trying desperately to run out time (see Inter v Barcelona's Champions League semi-final back in 2010).
As for the sending off (link here) we can see here by this shaky but multi angled video Vincent Kompany leaves his feet. But his right foot that wins the ball is not studs up, it is side foot with a slight passing motion of his right foot. His left is delayed as at no point makes contact with Nani. I think the last shot of this video is the telling one, Nani immediately gets up to try and win the ball back from what he deemed to be a fair loss of possession. The rest of his team mates also play on, trying to retrieve possession. Only one player reacts (Rooney) who immediately turns and demands a sending off (two fingers held up, indicating the challenge).
We always hear about contentious decisions and how to judge them. The truest test I believe are the players reactions: Nani and nine of his team mates play on, Rooney doesn't. I think the key to judge the illegality of any challenge is by gauging the reaction of the layers on the pitch. A horrendous, even ball winning two footed challenge will rightly draw a furious reaction from players. This one didn't draw any reaction at all, apart from Rooney.
As for the individual performances, Aguero was sensational. His work rate, his availability, his hold up play were impeccable. We knew Milner has an engine (and Richards too). But with ten men the onus falls upon the forwards and not more could have been asked of him. As for the best performance I have ever seen live I think I agree with Danny (who I was sat with) and Berbatov was sensational: movement, strength, intelligence, hold up play and close control of the like I had not seen since van Nistelroy (who was excellent at holding the ball up and taking away the defenders ability to strip him of possession by using close control and strong lower body). United promptly signed him on the back of such performances.
He was just as prompt in forgetting how play at such a high individual level. Of interest Spurs had ten men that day also. It was also the last time I recall watching live a player whom I also thought would go on to be a very special midfielder. Michael Johnson. My generations Paul Lake in a more self inflicted way.