In conversation, 저 is used for formal speech, it doesn't sound too formal, it sounds natural with -요 ending style, however, 당신(when it is used as a meaning of "you") isn't used that much I think, maybe just for translated sentences from other languages like "당신은 누구십니까?(who are you?)

For me, I use both of them, 너 and 니 equally. Did you do that? 너/니가 했어?

I won't 'answer' this because I'm not sure, but this is currently changing in Korea, I think. In dramas and on TV, it seems like Koreans are very careful to use either 나 or 저 (formal) when talking about themselves. However, they usually use the person's name or title rather than 'you'. In daily life, though, I hear 너 used quite often, or 니. It's confusing to me, so I'll be watching this to see the answer as well. It seems like there isn't a very clear answer right now in Korea.