Maybe? Most likely a buise or injury; however, do not take a risk. Get checked out by a board certified dermatologist. If it is a melanoma, the sooner it is treated the better. Can't tell for certain without an examination.
Possible. It could be. Time to see your friendly dermatologist.
Unlikely. Melanoma under the nail usually runs horizontally, or the width of the nail, and not vertically. The parents of melanoma can vary quite a bit between patients, so if you are still worried or the black area does not appear to grow out of the nail you should visit your doctor for a complete examination.
Not likley. Melanomas are not linear. I suppose you could have a melanoma under the nail, but I still do not think it would look linear.