Reading on Construct Armor modifications for building and modifying constructs, from what I am reading:
1. the construct is unable to perform any actions other than those constituted by the user, such as commanding it to attach or deattach.
2. Any attacks the user would sustain are directed at the wearer damage the construct.
3. Construct Armor counts as brestplate armor for purposes of determining AC, weight, dexterity modifiers to AC, and chance of arcane spell failure.
With this said:
1. If the construct would take damage and it has something such as Hardness, is the Hardness applied to preventing damage that the construct would take?
2. Similarly, if the construct itself had features such as magic resistance, and the like, would those be applied?
3. So essentially the construct would give an AC of +6, Max dex bonus of +3, armor check penalty of -4, arcane spell failure 25%, ect? If so, how would that interact with the constructs normal AC, if any interaction? What about other aspects that the construct may have, such as its hardness, or perhaps the Rune Carving of Rune of Shielding?
1) Most constructs don't have hardness, they have DR as do just about all creatures. There is only one exception I can think of that would regularly come up in regards to this, animate object (and psicrystals if 3PP is allowed). Hardness is typically the domain of objects (or broken/destroyed constructs I guess).
2) If something targets the construct you should treat it as normal. If a construct is in the area of a fireball and gets healed by fire damage, it gets healed. If the construct is immune to spells that allow SR, it is immune.
3) It doesn't have anything to do with the constructs AC, it has to do with targeting the wearer and the penalties for 'wearing' the construct. If you are targeting the construct you would be using the constructs AC. Seeing as the construct gets hit regardless of whether someone targets the wearer (per the construct armor) or the construct, the rune would trigger off a successful attack.