Ok a couple things.
If the construct is just a normal steel construct, what is it resistance? Or is it basically just like wearing plate?
They don't get resistances out of the box. Their Type covers all of the basics.
When it says critical "on a result of 51 or higher" is that the damage amount or the attack value? So, Unborn cannot be criticalled in the normal manner (roll in threat range and spend AD)?
Critical injuries involve doing over 25 points of damage. If you cause one, you roll an additional d20 and add it to the result to see what happens. The Keen quality adds to both rolls.