The guy to beat G is just a really really good fighter who like OP says can avoid most of what G dishes out, and absorbs what does get through, while having their own high level traits that will be effective. I think G is of the level that he will steamroll the lesser tier guys, but is totally beatable (as most are) against guys his own level. Those level of guys are usually rare at middleweight though, which is why only one rules at a time there. What I mean is, is at middleweight, I think in the last 15 years only Hopkins would be a sure fire favorite over him, and Pavlik would be a very very iffy fight one in which I could see a prime Pavlik winning, everyone else I think gets worked imo. So when G loses I think it will be against the next king of the division, whoever that may be who comes along in the next 5 years or so, or when he moves up and neutralizes his attributes (like his haters are praying for him to do) That being said, I do think hes vulnerable to a guy like Lemieux, though I don't think he will lose if he does have some rough spots it wont surprise me either.
Boxing is about levels, and being able to apply skills against second and third tier opponents doesn't mean it will be effective against first tier, something Golovkin has never fought as a pro. With all due respect, if his team were a fraction as confident as you on how a matchup between him and Ward would go, they would have made the fight happen by now. An impressive Ward win would do more for his progression in just about every aspect of the sport (hype, legitimization, credit, valuable in ring experience, future leverage in negotiations, ect) as a 100 Monroes and Murrays.