Don't know why you think Abby is a proven centerman. I think the last time he played center was AHL so that would make AA as proven as him.
I think centermen always have more defensive responsibility than wingers. Wingers go along the boards and get caught up with players. Centermen are in free ice and so they're the logical/reliable ones to skate back and help the D. Because of this, it is the place where the fastest skaters play because they have to go north to south. To me, having AA and Larkin at center, is how we make use of their speed.
I think we'll still see Z as the top center, which I think is fine, but hopefully Larkin gets better at faceoffs and takes over as the year goes on. I'd like Helm as the 3rd line center - He would certainly help the faceoff % - but I realize that blocks either AA or Sheahan so I know it doesn't fully make sense.
To sign another centerman (unless it's Stamkos - i've heard he wants to play center) at this point just gets in the way of the future. I even think signing Richards to play center (though he played well at the end of the year and playoffs) this past year was overly cautious. We're not winning the cup next year (barring some big moves...maybe Stamkos, some surprise big trade on D and a lot of internal development) so we're continue this youth movement and get things going in the right direction for them taking over the team.