1. All this 3D and 3D is getting confusing so just please just use stereoscopic for the one that now doesn't need glasses and 3D for the one of Mario64
2. A 2D(eg. Side scrolling) stereoscopic game is possible and Nintendo has also stated that the stereoscopic mode can be turned off
3. The 3DS is the end of the DS just like GBA was to GBC, because is not a redesign. It is the successor of the DS
Gen I
GameBoy -> GameBoy Pocket
Gen II
GameBoy Colour
Gen III
GBA -> GBA SP -> GBA Micro
Gen IV
DS -> DSlite -> DSi -> DSiXL
Gen V
3DS
4. Because no.3 the 3DS games cannot be used in the DS but DS games can be used in 3DS but without the stereoscopic mode.
5.Sorry if some of the above have been already posted. tl;dr
P.D. E3 begins tomarrow!!! but Nintendo's Conf. is on Tuesday
P

Just hope all the rumours about the Control Sticks and stereoscopic cameras come to be true