If you'd like a simple answer for the shield-and-back question, consider this:
If you think you should get the +2AC for having a shield hanging on your back, then why doesn't everyone, literally everyone, do that?
It could be why we are limited to two weapon slots only and why they have to be melee and ranged.
While for anyone not relying on melee having a shield in their melee slot is beneficial, one cannot dual wield or have a two handed weapon while benefiting from shield.
