Well, it can also be a possible leaking or electrically shorted fuel injectors. (nissan injectors are prone to lots of problems, by the way) Someone else thought that the extra fuel came from the fuel pressure regulator, and the catalyst got fried from too much fuel, and the MAF sensor changed nothing.
So, we have 3 witnesses that say "there is too much fuel!".
The fuel injectors and the injector circuits to such must be investigated. Next, the coolant temp sensor must be checked. Before condemning the ECM, you must check power and ground circuits to it. They just don't fail that often, so I will be surprised to see it end up testing bad. Nissan calls a MAP sensor a boost sensor. It allows for altitude compensation....check it, too.
A scan tool is a must if you need to figure out what the computer is commanding versus what is really happening. A generic scan tool on a nissan is typically not very helpful. Nissan is rather secretive about its computer data, and you have to have the right software to gain access.
Let an expert condemn the ECM, but never an amateur. An amateur can't refund you the money if it is a mistake.
I still wonder about those injectors. If one is stuck on mechanically, that can foul out the plugs and converter. If one is electrically shorted, the "on" time will be irregular, and could cause the computer to freak out from the excessive current draw. If the injector coil resistance is too high, the injector may be sluggish to respond physically, if at all. Nissans don't seem to admit there is an injector fault even if you unplug them, and I hate them for it.