Heh, read closely, children (do I sound like a school teacher or what?)
It's not "+1 immunity to all weapons and less", it's "immunity to all +1 weapons and less". Big difference. As you already figured out, it only protects against normal (nonmagical) weapons, and weapons of enchantment level 1 (like a longsword +1 etc). So it doesn't protect against +2 weapons and up, and it's indeed not stackable.