From my own experience towards a Martial art's master from Busan. I was told to use 당신 towards people with more status. In Seoul some people don't care, but In Busan.. well, the master was confused, and said "no no no no" he grabbed my shirt with his punch and rised his right arm like he was about to punch me in the face. And said "Dangshin!!" So I understood it's not a way to approach someone ^^U
In English it is normal to use pronouns rather than using the name of the person over and over again. But in Korean, you can use "you" by using the title of the person or the name of the person.
Example using name: 진수 씨 지금 뭐 하고 싶어요? (Jinsoo what do you want to do right now?) You indicate 씨 in every name to sound polite. And if you use this every time in every sentence to say "you", it's okay, it's normal in 한국어.
Example using title: 선생님 이거 뭐여요? ( Teacher, what is this?) There titles such as 아저씨, 아저마, 학생, 엄마, 아빠 etc.
If you don't know the position or title of the person: Omit the subject of your sentence.
I suggest you go learn more about this in "Talk to me in Korean" they have a lesson dedicated to the word 당신 and when and where to use it. I hope this helps!
Foreigner. I've never heard a Korean say 당신 before. They either say 너 (informal) or say their name, even they're right in front of them. If you're talking to 서빈, you can say 서빈 씨 안 먹었죠. = You didn't eat, right?
If you want something for yelling out to someone, you would say 저기요!
I think it depends on whom you say it to. 당신 is considered rude, especially when you say this to a stranger or people you have no romantic relationship with. But it also has poetic meaning, hence you'll find it a lot in love songs. Some couples (usually married ones based on what I've watched on kdramas and Korean vlogs) call each other this other than 여보.
I've heard that you need to be careful with this, but when I was in Seoul I used it toward my host parents and they didn't seem to mind, and I'm pretty sure I've seen it used elsewhere. But also I'm an obvious foreigner and Korean people are generally patient with foreigners, thankfully! I wonder what is more likely to cause insult, a misused 당신 or a misused 너?
A misused 당신.
당신, if misused, may likely be a way to address someone without any consideration in honorifics and title of the person. And it might come of as "intentionally" wanting to fight that specific individual you called 당신.
If you want to learn more about the proper usage of 당신, I suggest "Talk to me in Korean." They helped me alot in explaining why 당신 should be rarely used, when can you use it, and what are the proper alternatives to say "you" in Korean, like addressing to title or name.
I hope this helps.