For the record NSMB had been fairly extensively hacked. Also it is a fairly good example for a lot of the commonly known DS SDK formats, possibly even the game many people use to learn about them.
https://github.com/Dirbaio/NSMB-Editor is the source code to the program for editing NSMB, some of the earlier work from previous authors should be a bit more human readable if you prefer.
By all means continue to pull it apart, you will learn an awful lot and there are not so many things to truly trip you up but know it has been done before.