4
Sorry but the current behaviour is not correct.
Every kind of forum system I have used so far does count the posts in the subforums (as well as the last post) when showing the total of the main forum in the index page, which is normal because otherwise it may seem that a main forum is empty whil there are actually several posts in the subforums it contains.
This way there is no way to know from the index page whether there have been new posts in the subforums.
Also I don't see what would be the problem of the pagination, as the count only is shown in the index page and it shouldn't matter which number is written there.