Exactly.
1. Scarlett in defense > Chad in defense (overall though Chad better as the rebounding defender)
2. Chad forward > Scarlett forward
3. Chad in midfield > Scarlett in midfield
So if I had the coaching job at a club other then Geelong or Port and both were up for trade at a low price and could only get one, unless my club desperately needed a full back, I'd grab Chad since I know I could throw him in the midfield or forward as well to plug a problem.
Your comparisons are unfair because Scarlett has not been used in the same positions that Cornes has. The reason for this is because he is SO good at playing his fullback/loose man role in defence. It's like comparing Dean Cox to Ben Cousins in terms of 'who is a better player', and then saying cousins plays the roving role better

Or more precisely, like saying Luke McPharlin is a better player than James Clement because he can also go forward... i.e. the only reason stopping Clement going forward is he is so damn good in his current role that it would be a waste.
On your points:
1. True, Scarlett is by some margin the better defender - yet you can't resist qualifying your statement by saying that Cornes is the better overall defender as he rebounds... spare us. Scarlett can rebound and can get 30 possessions whilst limiting a gun forward, which Cornes can't do. How can you possibly say Cornes is the overall better defender when he can't actually defend. He is simply a loose man who has been given a license to run out of the back half by his coach, similar to Joel Bowden. How would he go lining up in the goal square on a red hot Barry Hall, Fraser Gehrig, Brendan Fevola or David Neitz? He'd have a bagged kicked on him without a doubt. Scarlett is the better player in defence by a country mile.
2. Chad cornes is the better forward, true
3. Midfield - who the hell would know, Scarlett never plays there. Once again, like comparing Cox and Cousins, or even like comparing players of differenet eras