So I'm no longer a sixth former, but I will answer what I remember:
1) Not sure how many I had each week.
2) We were allowed to leave my sixth form during free periods, which I did a fair few times. That said there's not a lot you can get done in an hour once you factor in travel to and from your destination, unless your free periods are at the end of the day or you have consecutive free periods.
3) I enjoyed sixth form, but I'd be careful about getting unrealistic expectations.
4) Maybe? This depends on your sixth form's policy on leaving, and on you being able to find suitable volunteering opportunities.
I think a lot of places tend to refer to them as "study periods" rather than "free". Although it's fine to use the time for administrative tasks and some extracurriculars, I think there is an expectation that you try to get some actual work done in those periods.