Hi,
Here are some answers:
=> No limit on the total number of members. But an organization is made of teams, and one free team must have less than 15 members.
=> An image is better than a lot of words:
=> Teams belong to an organization
=> If the member is outside of the premium team then he will be considered as a “limited access” member, and shoud not count into the limit of 15. Am I right @Marie ?
=> No the premium features are only active in the project of the premium team.
=> You won’t be able to do exactly that, according to my previous answers. Premium features are only for projects belonging to the premium team.
Hope this helps.
Julien