Belariath

So, there are several pieces of armor that have an inherent stat bonus. For instance:

Robe of the Arch Magi: Res +2
Circlet: Int +3
Shadow Weave: Agi +2
...
etc.

Are these considered an enchantment (i.e. do they take up an enchantment slot), or just part of the armor's inherent properties?
Also, if you were to enchant the items with that particular stat, is there a limit of +5 in total for that stat (so a circlet could only be enchanted with a further +2 Int), or +5 on top of the automatic bonus?

Yes, those are considered enchantments.
Yes, there is a hard limit of +5, period. The enchanting you perform upon it is IMPROVING and the de-enchanting RESETS the enchantment that the armour comes with. Think mithril weapons and their inherent +1 ATK enchantment

Additionally, as said de-enchanting the armour resets the stat enchantment (which is a permanent stat enchantment by the by) to their fresh from store amounts, there is no way to remove said enchantments completely from an item.

Thanks.