He played 12 four times last year and 13 twelve times. I understand why you'd want him at 12 but surely it's like all team calculations: What do I get from this combination v what do I get from that combination.
9,10,wings and fullback left alone, what do you get from Ellison and Inman? You have the centre who was keeping Nonu out of 12 for a few games last year and started all 16 games for his team and you have the 5th choice 13 in Australia who is consistent and plays in a creator role more than a finisher.
So Ellison at 12, Inman at 13 is having two creators in the centres, not pushing either out of position. Both can do everything their position requires well. Inman at 12, Ellison at 13 still has two creators in the centres, but is pushing Inman out of position to accommodate Ellison at 13. Inman may not be able to perform every job required of 12 as well as you need.
Really I just think Ellison at 12, Inman at 13 puts a lot less strain on the team while also letting your talented new signing touch the ball more often.