1.Who created the ReiNAND fork known as Reisix?
2. does CTRmus work on Reisix?
3. Why is there no System Shock 1 port for the 3DS? I know that the game makes use of the mouse a lot, but can't that be remedied by either configuring for the touch screen, or even binding the N3DS 2nd circle pad?
4. Could it be possible to sourceport undertale to the 3DS?
5. Can .3dsx files still be played on a CFW 3DS, or is it better to stick to .cia files overall when not playing games on a physical cart?
As for your last question. 3dsx files aren't commercial games at all. .3ds are used on carts, and .cia on cfw - you can't run .3ds without a cart.
3dsx = Homebrew
3ds = commercial game (warez)
Cia = installable format