It isn't ok for anyone to open anyone else's mail. Apart from being a criminal offence, it is an invasion of privacy.
Yes
Yes, with parental permission.
Certainly, if they have the parent's permission.
No, you cannot.
No
with parent permission, i think so.
no
Not legally.
Yes
Without their permission they will have to wait until they are 18. The parents are still responsible for them.
Yes, if they have parental permission. That does not relieve the parents of their responsibilities.
No, you are a minor.