I use to do alot of chimney work in my contracting business so I'll try to help, the damper sits on the frame and hinges on a couple of cast ears, if it's like all the ones I've ever seen in the last 30 years the pins should just be for the handle/lever linkage, you can start by removing the handle by taking the pins out, then try and push up on the damper on the upper edge, it hinges on the bottom edge.
if you can get it to budge a little upwards an inch or 2 then you can slide the bottom/back of the damper forward off the back of the frame, then when it's off the back you will still have to slide it to one side or the other to get the back corner to drop down. you'll have to get under it to do all of this so be real careful cause your gonna get a lap/face/head full of brick and mortar.
you might have to use a couple of pry bars to move that damper with the rubble on top. it's possible that it's so wedged in there that it's not gonna move. though I've never seen this and have done a few lightning jobs.
Worse case #1 check your dampers style/size with a new one at the local mason supply yard, with your home being so new they may still have the same one and you may be able to get just a door since the frames sometimes come in broken on account that they are a thin casting, then you can break the damper with a heavy hammer it's also cast and will break pretty easy.
Worse Case #2, you'll have to open the chimney from the outside above the height of the damper to remove the rubble. if this is an insurance job then the adjuster should be made aware of the problem.
Good Luck, JB.
