Can low sodium cause naseau?

Yes. Hyponatremia (low sodium) can cause nausea. See medline plus article on this topic. (http://www.Nlm.Nih.Gov/medlineplus/ency/article/000394.Htm).
Yes. I would say the cause of low sodium would probably do so, rather than the low sodium itself.
Yes it can. But there should be also some other symptoms of hyponatremia, such as vomiting, headache, confusion, lethargy, fatigue, loss of appetite, restlessness and irritability, muscle weakness, spasms, or cramps, seizures, and decreased consciousness or coma. The level of sodium in the blood to cause these symptoms should be below 125-135 meq/l.