Yea, those are my thought exactly, nothing wrong with Ai-chan being sub-leader, its how the system works, and its not like a sub-leader has big duties anyway (neither does the leader, beyond keeping it friendly with the girls it seems and tv talk)
Yes, basically a leader's job is just to keep the motivation within the group up, and ensure that everyone is focusing hard on their practices and such and of course, to help keep peace between everyone. Aside from that, to try and talk the most during talk shows, etc.
A sub-leader's job, you could say, is just to be there in case the leader is gone; it's like one is already assigned so that they don't have to take up time thinking about who the new one should be in the future

A sub-leader hardly does much, and is just like a regular member, who'se also required to talk a bit more besides the actual leader.