To get it go through the add and complete it. then when you get out of the vr machine go to the vault where everyone is gathered and open it via the terminal it should be in the middle of the vault. it isn't actually in the add on its a reward for completing it. :)
i dont know what you mean by "PA", but the winterized t51b Does degrade, i heard from fallout.wikia.com (use this site for any Fallout Qs) that the developers messed up and gave the armor the HP of the simulation version so it has like 1mil health.
After you finish the main quest the game ends. But if you own it on Xbox 360 and have Xbox LIVE, Broken Steel will extend the main quest and will raise the level cap from 20 to 30.