I think that there are a couple of points here that I could elaborate on, but I don't claim to know everything either, even if I have spent more hours on the forum.
Regarding activity, I think that if you are averaging more than 14 posts per two weeks, then you will get all of the 14 activity points for each activity period even if you only post a couple of posts in the most recent activity period. However, when you start to go below a 14 posts per two weeks average, then you receive fewer activity points. Something like that. So I have mostly kept my average up above 14 per two weeks, and I think that there was only a few two week periods that I did not post anything, so I did not receive any activity points for those few two week periods.
Regarding activity, I think that if you are averaging more than 14 posts per two weeks, then you will get all of the 14 activity points for each activity period even if you only post a couple of posts in the most recent activity period. However, when you start to go below a 14 posts per two weeks average, then you receive fewer activity points. Something like that. So I have mostly kept my average up above 14 per two weeks, and I think that there was only a few two week periods that I did not post anything, so I did not receive any activity points for those few two week periods.
As you already suspected, you are wrong on this one
You can find the calculation for activity here
The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Lets say you have a new account which does only one post in his first two-week period, this account would have an activity of 1 but his time variable would already be one which results into a potential activity of 14 for his account.
So if this account would make 27 or more post in his next two-week period he would have an activity of 28.
Therefore its enough to make at least one post in one period to maintain an increase of your activity but to actually make it increase you would have to make many more posts.
And sadly thats the case for my account
I dont even know my potential activity. There has been a website which was able to calculate that by using your post history I think, but it is no longer online.