I'm not sure that I follow what you are trying to do here, and whether this is the best approach.
When someone creates an Activity Feed "Post" record, who do you want to see this? Everyone?
First question: why? Part of the beauty of Activity Feeds / What's New is that it shows the information that is relevant to me. So it is filtered to only show posts about records that I follow, or written by people I follow.
Assuming you don't want to filter it at all, and see all the posts in the whole system, then you could dispense with the What's New activity wall and just display a list of posts - but this won't give you the inline controls to add a new post etc.
Second: your workflow is following the thing the post is regarding. This makes sense, but does mean that very often you will be trying to create a follow that already exists. In earlier versions (I think even up to 2013 RTM) this would fail because it would not allow a duplicate record - by only allowing one, it makes sure the "Unfollow" works. This does now seem to be fixed, and uses a try/catch sort of approach to create a new follow if needed, or just leave the existing one if not, so the workflow does not fail at that point.
Third, the owner of the follow is the one that it applies to. As far as I know, you would have to have the owner of the follow be a user, but I must admit I have never tried this with a Team, it might work as long as the Team has appropriate privileges.
A possible alternative approach would be to make sure that all users follow all other users. This would mean far less work going on during the working day every time someone posts something to the Wall, and you would see anything posted by anyone.
If you have a really small number of users, then each user could just do this by manually following everyone else. To automate this you would need a custom workflow assembly (or plugin), and could then run this retrospectively, as well as when a new user is added to the system. Logic would be something like: With this [new] user, for each "other" user in the system, add a follow record regarding the current user, owned by the "other" user.
