Amulet of Natural Armor = enhancement bonus to Natural Armor to AC.
Bracers of Armor = Armor bonus to AC
Ring of Force Shield = Shield bonus to AC
Ring of Protection = Deflection bonus to AC
All of those stack with each other.
Mage Armor = Armor bonus to AC, this does not stack with Bracers of Armor because both are Armor bonuses to AC.
An invisible but tangible field of force surrounds the subject of a mage armor spell, providing a +4 armor bonus to AC.
Mage armor provides a +4 armor bonus to AC.
It's a named bonus "armor" so wouldn't stack with other armor bonuses. In this case, your bracers of armor +5 (it says it's an armor bonus).
No, the general rules are sufficient for this.
Bonus: Bonuses are numerical values that are added to checks and statistical scores. Most bonuses have a type, and as a general rule, bonuses of the same type are not cumulative (do not “stack”)—only the greater bonus granted applies.
Stacking: Stacking refers to the act of adding together
bonuses or penalties that apply to one particular check or statistic. Generally speaking, most bonuses of the same type do not stack. Instead, only the highest bonus applies. Most penalties do stack, meaning that their values are added together. Penalties and bonuses generally stack with one another, meaning that the penalties might negate or exceed part or all of the bonuses, and vice versa.
Armor/Shield Bonus: Each type of armor grants an
armor bonus to AC, while shields grant a shield bonus to AC. The armor bonus from a suit of armor doesn’t stack with other effects or items that grant an armor bonus. Similarly, the shield bonus from a shield doesn’t stack with other effects that grant a shield bonus.
Others have given the rules already, here's a bit of explanation.
While it might seem fine to stack Mage Armor with normal armor, but allowing Mage Armor to stack with itself (which you're almost forced to do if you do the first part) is not such a good idea.
That high-level wizard with a Ring of Wizardry I? Prepares 5 Mage Armors, boom. +20 armor bonus to AC. That applies to incorporeal touch attacks.
Your guess about spells stacking if they don't say they don't is actually wrong; I believe there's a section in the Magic chapter that says multiple spells with durations don't stack (Mage Armor/Resist Energy don't stack, whereas instantaneous ones like any Cure or Fireball obviously have their effects overlap).
No, you cannot stack Mage Armor with regular armor. Both are "armor bonuses".
Armor enhancement bonuses are to the armor bonus the armor provides and are not a separate type of armor bonus.
Enhancement Bonuses: Enhancement bonuses apply to your armor to increase the armor bonus it provides.
So no, you cannot stack +1 Leather and Mage Armor and count the leather armor's enhancement bonus while using the Mage Armor's AC bonus.
The total bonus would be a 4 armor bonus (the Mage Armor's bonus completely overwhelms the Leather armor's bonus).
However, while the bonuses don't stack, Mage Armor does overlap regular armor against Incorporeal attacks.
For example:
Rogue A has an 18 Dex and wears a Chain Shirt. That gives him an AC of 18.
Rogue B has the same, but also has Mage Armor in effect. His AC is also only 18.
When an Orc attacks either Rogue A or B, the Rogue's AC is 18.
However, when a Shadow attacks Rogue A, it is against an AC of 14 because he bypasses the Chain Shirt armor. Against Rogue B, it's still an 18 because even though it bypasses the Chain Shirt, it doesn't bypass the Mage Armor.
