Greaseproof paper is usually for cooking and is coated in an extra substance so you don't get greasy! As long as the lines of what you want to trace are dark enough, greaseproof paper will work, just makes images harder to see if they have faint lines.
Yes, Most of the time. Except grease proof paper is normally thinner. Tracing paper has a thicker texture. But if you want to save money, grease proof paper is a great idea.