Gamer tag and personal

If it was changed by Xbox, then that is typically a forced gamertag change given the enforcement team because the tag violated the code of conduct. You can check http://enforcement.xbox.com for information on why it was changed and to see about submitting a case review.

If he is just threatening then he can't hack really. He is just trying to scare you. Also don't name and shame on the forums. Just file complaint and move on.

No. There is no way to get a free gamertag change.

If the gamertag violated the code of conduct, you could get a forced gamertag change from the enforcement team, but they will only give you a randomly generated tag, not one you choose yourself. And while having personal information in your gamertag is not
recommended, it does not violate the code of conduct. So you will need to pay to change your tag.

I have seen a person with a green clan tag and gamer tag in a ffa

My understanding is that gamertags aren't limited to one person any more...if someone else already has the same gamer tag, the system simply adds numbers after it to denote the difference....so, for example, if "Example" was a gamer tag and 2 people used it, then one person might be "Example1064" and the other might be "Example1654" (no clue how the numbers work)

Whilst you may have deleted the account with that gamer tag, someone may have used the same gamer tag since the system was changed