As to airlines not carrying out their own rules, we have all seen the pax in the bars who are just waved on board to create problems later. The gate staff just want rid of them and the CC know the problems of turning them back at that point. We all know how these people can 'sober up' when they have to present themselves.
So you're claiming that the gate staff and cabin crew know the pax are pissed and are letting them on, but at the same time you're saying that pax are acting sober enough to board (in which case it can't really be the gate agent / cabin crew's fault). Which is it? Forgive me for saying but you're on something of a meandering rant and I'm not sure if I know what your argument or your real point is any longer.
For what it's worth, and as someone involved heavily in this issue, I can tell you that most pax that end up being disruptive (I'd say around 90%) don't board in a drunken state and aren't showing signs of potential disruptive behaviour at boarding. It usually starts later on in the flight when they get stuck in to the duty free, and when altitude makes the state worse, or when they simply fall out with another pax or a crew member. It's very hard for crew to police that problem proactively, but I can assure you that airlines are not routinely turning a blind eye at the gate and neither are handling agents, although clearly there will be an odd exception where that does happen and we deal with that on a case by case basis. Why would any airline encourage crew to take the problem into the air, considering that the cost of a divert can be anything from 8 grand to in excess of 30 grand when it gets complex with FTL issues etc?