I thought there were some importers for blender and such.
Equally even if there are no nice graphical editors then most models on the 3ds are simple enough you can still do things by editing the component aspects of the models, and if there are viewers then edit, save, refresh viewer.
Re Nintendo dev tools. The main problem is to join it you usually have to sign an agreement which tends to preclude you from working on homebrew or using them for hacking purposes, and said agreement goes into the future as well.
They are unlikely to come kick down your door, or DMCA your project, but it is a real thing to consider in this, and some may not want you on their projects/emulator coding efforts.