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?
- After all, you only need to be proficient to wield a shield - you don't need to be proficient with it to simply hang it on your back and *not* actively wield it.
- So, wizards, dual-wielders, monks and everyone can do this, if you get the AC from having it on your back. It doesn't interfere with the Wizard's casting, because they aren't wielding it. It doesn't block the monk's armoured defence, because they aren't wielding it, dual-wielders don't need to give up a wielded weapon, because they aren't wielding the shield.
- This being the case... what's the benefit of actually wielding a shield? There literally IS no benefit to wielding a shield now, because you get all the benefits of wielding the shield for not wielding it and leaving both hands free. Proficiency with shields is meaningless because lack of the proficiency means nothing, since you get the bonus while not wielding it.
Does this sound sensible to anyone here? Because it shouldn't if you're a rational person.
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If I have a rogue trying to back-stab someone with a tower shield (which is just a shield - there's no distinction of shield grades in 5e) on their back it's not going to fail because of that shield - that shield blocks the blow from the front because the individual wielding it is actively doing so and shifts the shield to block me when I try to stab around it... This shield on the back is not being monitored or watched, and it's not being wielded by an intelligent entity - it's something that I, an intelligent entity can see... so I'm not going to stupidly ram my dagger into the shield. I'm going to reach around the shield and stab the person in the kidney... because the shield, when not being actively wielded by an aware and intelligent person to stop me, is functionally doing nothing.
