1. If its free for you to try its likely not any gram amount, grams of coke can cost $40-60 , it may be a couple hundred mg.

2. Not that I recall (probably), im sure it would also depend what kind of cuts are in it (average purity of retail street coke was 56% in 2002) http://www.unodc.org/pdf/WDR_2004/Chap5_coca.pdf

3. I'd say try a line and depending on how you feel ten minutes later do the others.