Q:

Could anyone with access to a nation's nuclear launch codes unilaterally fire nukes as they please?

If someone were to have a country's (Take the U.S, for the sake of being specific) nuclear launch codes, could he unilaterally fires nukes as he pleases?
Assumptions:
1) He's among the military's top brass and can, conveniently, access practically any room in the White House without questions being asked.
2) He's got a loyal team of about a dozen people at his disposal, so he doesn't have to do everything himself.
3) He has access to the President's "nuclear football", and managed to obtain a copy of the launch/verification codes.
4) He decides to fire the missile(s) on the same day he obtains the codes, so even changing the codes on a daily basis is no impediment to his plan.
5) He already knows where a particular launch site is.
6) He's got 10 minutes to get into the Oval Office/Situation Room, punch in the codes and get out before anyone suspects anything/ walks in.

A:

No, after ordering the strike the Pentagon war room askes a verification code over voice communication. Without that verification code by voice the strike will not be initiated. So unless he remains long enough to make that call it won't happen. If he can, it will move forward. 
However that verification order is send not just to the launch crew but several command centers as well. With still 3 minutes to go before launch those command centers will know a strike is initiate and can contact the launch site directly as that location is embedded in the order.
Submarines take an additional 10 minutes to fire. Although I can't tell you exactly why. Their launch procedures are longer thus would give even more time to prevent launch.

A:

As with most of the other respondents, I am going to say a firm No.
Let's walk through the entire scenario.  You have a high level adviser with a team in place in one of the most secure areas in the United States, presumably with access to the President, Vice President, and all of their staff, but the President is not part of this conspiracy and will not cooperate to launch nuclear weapons.
Your first hurdle is getting the authorization codes you need.  It is presumable that you could replace the "Football Carrier" with one of your people, but the football is only one part of the recipe to launch Armageddon.  To authorize the attacks, the President also has the "Biscuit," a gold credit-card sized piece of plastic that must be physically broken to reveal a code used to authorize the attack.  Without the President on board, you couldn't authorize a launch.  The President's identity is also verified by SecDef, so unless SecDef is in your conspiracy, you have that hurdle as well.
Let's sidestep that issue.  Say you incapacitate the president, and have the VP on your side, using his football and his "biscuit" to authorize the strike, or managing to give the real President sufficient evidence that he believes a strike is needed.  You send out the nuclear launch command to NORAD.  Various nuclear facilities are activated, verified, etc, and prepped for the launch.  Given that we have never gone to the brink with nuclear weapons since WW2, it is possible that operators will refuse to launch.  Just from sheer probability, some will fail to launch for various reasons.  At least a few weapons will launch under any scenario where the President (Or VP with the President out of the way) authorizes a strike.
However, it would probably be easier (with the military connections and access you have) to secure a "Boomer" sub.  Russia is almost guaranteed to counter-strike if they are attacked, and one the nukes are in the air, the US will have no options left but to strike back or die quietly.  

