how is och pronounced? i read through the discussions and have found many differing answers.
You can pronounce is like it's spelled too, which is more like the "oc" in the English "doctor". The "oh"-sound eventually comes automatically when you speak fast, like " 'cuz" instead of "because" in English, or " 't" instead of "het/dat" in Dutch.
Not really. greetings can have many meanings, but I don't think hej fits any of them.
Is välkomna still widely used or has välkommen took its place for addressing to more than one person?
Would I be correct in assuming this phrase would have very limited use? Such as greeting a student or intervewee in a one-on-one setting...
I think it's too narrow to say that it would be very limited. I could very well say this at my work as a receptionist.
Is there any diffrence between "hej" and "Halla" as both of them could be translated to hello
I gather that the difference is context and formality. Hallå has more emphasis or surprise on it (Imagine you're saying hi to someone but they don't hear you - 'Hej! Hej? Hallå!'), and the example I keep seeing people use is answering phones (a bit more formal, but not necessarily standing on ceremony).