resetting the CEL is usually done with some type of interface software (laptop etc) but sometimes there are manual methods of doing it, like unplugging the battery for a bit. just because theres a CEL doesnt mean that there is a current problem, sometimes it just needs to be reset. you could probably go to a nissan shop and they would do it for you.

any time you open your MAF to the open air, you expose it to anything and everything floating around. theyre quite sensitive, so that couldve pissed something off.

if your car still runs fine, id have to think that the MAF isnt broken and it doesnt need to be replaced (if it was broken the car would run horribly) and replacing a MAF isnt quite the cheapest thing.