Indian guy here (specifically from Pune) been here a few years and fairly well adjusted to American culture (married to an American girl).
1) Yes, it is true that people have sexual relationship "very casually", however it's only relative to typical Indian frequency, which isn't a fair comparison. One thing that has helped me understand the US culture is avoiding comparison with your initial bias. No culture is a golden standard, so it's a bit like comparing currencies. Haircuts for example in the US cost typically around $10 - $15 which sounds ridiculous when compared to $0.5 that I paid in India. But that's an unfair way to look at it because of several socio-economic factors. So personally I've found the re-orientation to be very useful.
As far as love and relationships go, I'd say sex in US is not considered as an ultimate goal like it is sometimes in India. Sex here is something you enjoy doing - like very expensive dining. You don't want to do it so much that you're broke, but it's also not something you do once in life and be happy with. People are still people and among the variety you can find someone you can trust based on the extremely people oriented skills you have learnt in Pune (that you don't realize you have right now). Test and perfect your hunches about people and eventually you will figure it out.
2) Yes, in general younger people are not fans of direct advice. You have to have a really deep relationship with someone to be able to directly tell them what they should do. Even then you should be careful. Instead, you frame it is a way that always gives them an alternative: e.g. "I'd just call him out next time he does something shitty." That way if they feel they're being told what to do, you can save yourself by saying "I was just talking about myself." Eventually you will start thinking this way and will not involve yourself in other's life as excessively as we do in Pune. You will feel freeness you have never felt before.
3) Yes, being overly friendly can sometimes (very often if you're a girl) be perceived as flirtatious. This happened to me early on as well when my classmates thought I was flirting with every girl I met. You will notice quickly that there are friendly things people say that are normal and some that are more likely to be flirtatious. For example, saying "thanks", "have a good day" etc to a barista is normal, but if you get overly creative like me and say "you should have a PhD in coffee" to an opposite sex barista, that could border on flirting because it's excessive. In Pune we are oriented towards friendliness and sex is a very remote possibility if at all (almost never), however, especially on college campuses in US, the orientation is niceness with a very real possibility of sex. So you have to have that at the back of your mind and trudge along. But honestly, it'll just be funny most of the time and nothing to really worry about.
4) You don't have to get along beyond being socially tolerant of each other. This applies to several other relationships as well. Most people have their own life (I have never seen my neighbor in 1 year that I've lived here which in Pune is impossible), and socially speaking you can just be polite and go back to your own life as soon as possible. You don't have to agree or laugh at their jokes etc. Typically nobody persecutes your for your beliefs or not agreeing or not laughing. People respect other's personal freedoms pretty well.
Bottom-line: Don't worry, give yourself time, trust in your people skills, you will be better than fine.