Bedford toughness should give you 3 hp, so that would bring your total to 20. Sorry I missed that.
I'm going to say that shields are balanced for size similar to the way that weapons are, so strapping a light steel shield onto your arm like a buckler would add to the ACP. Besides, it gives the same armor bonus, why not just use the buckler.
Walking_Dad is right, you don't get the benefit of the buckler when using your two handed weapon. It stays on your arm, so it's there if you switch weapons, but you don't get the bonus unless your off hand is free.