Can somebody PLEASE explain the difference between this ultimate "forwarder" and the ultimate Injector release?
The "Injector" takes your ROM and packages it into Nintendo's own SNES emulator that it uses for official SNES virtual console games. The good things about this is that compatible games typically run very well and it uses the exact same interface as the official SNES VC games for pause/reset/restore points. The bad thing is that not all games work with it, such as games that use special chips like Star Fox, Kirby Superstar, Mario RPG and the like. I think it also can't run Star Ocean, and Dragon Quest III, simply because the ROMs are too big. And, of course, N3DS is required to run them, just like the official SNES VC.
The "Forwarder" packages your ROM into an emulator developed by the community instead of Nintendo's official one. You could just install an emulator as a CIA and put your ROMs into a folder, but this option is self contained and looks more official. It looks like the real thing on your home screen and it has save-states like the official one, but there's less polish. The pros are better comparability with special chips and huge ROMs than Nintendo's option. Also, some emulators will run on O3DS, but this forwarder doesn't currently use any of them. However, compatibility still isn't perfect. For instance, Mario RPG still has some graphical glitches in the background and Kirby's Dream Land 3 renders water as opaque, so you can't really play underwater sections... at least until a new version is released that might have more compatible emulators.
Injectors are currently available for NES, SNES, GB, GBC, GBA, Game Gear, and TurboGrafx, all using official VC and boasting mostly excellent compatibility. Forwarders are currently available for SNES and Sega Genesis.