if an account has been inactive for a while like 8 weeks, if it starts posting again can it get the 56 activity it could have got in the time or only 14?
It doesn't work like that, and for good reason. If it did work like that everybody could create 10+ accounts and then just let them sit for a year or so and start posting again and become a Sr. Member or similar instantly. You have to post during every 2 week period to get the activity increase.
That was my interpretation of it at least. Each post is worth 1 activity, up to a cap of 14 which is reset every ~2 weeks. There is no banking of points or potential mentioned anywhere.