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.
That's giving Larian a lot of credit. It's vastly more likely that Larian simply decided that the way people'd want to play D&D/BG3 is with one set of melee and one set of ranged weapons per character, and they didn't consider (or think important enough) the consequences of being able to swap freely between those sets or that some people might want 2 different melee sets.
I highly doubt that they created BG3's (simultaneously both too restrictive and not restrictive enough) weapon slot system with the intention of restricting shield usage specifically. After all, their implementation still allows ranged characters to get bonuses from a shield every turn which is a free +2 AC; why can only ranged characters benefit from this?
