Don't really understand what you are asking in question 1. Our system gathers the data and tells us which clans are inactive/active

For question 2, no, half of the players in a clan have to be active to get full points for in-game activity regardless of member count

Question 3: Technically if a clan has 20 members, 5 of which are active, 15 inactive, then if they kick 10 of the inactive members, suddenly half of their clan is active and they get more points for ingame activity. Since forum activity is just based on posts on the board in a month, a clan's member count doesn't have any direct effect on that.