The white screen bug will be fixed in the next version, as it'll be able to boot TWL cartridges (only in DS mode).
No, this is not intended, otherwise, the screen wouldn't be in widescreen at all.
Widescreen actually works fine for me with cartridges.
What theme are you using?
I've tried all the themes and it doesn't seem to work right in any of them. In any case normally I launch the game from the DS Classic Menu.