Forge, much of the wall is still covered by tapestry (click on the map a few times...), the wall you search on the southwest seems normal. Your guess is that you being in the room triggered it, and it will stop sometime after you exit.
Buckthorn, you are able to walk over, get the coffers, and walk back....
As the chair strikes the dull aquatic scene, it transforms into a great slithering green blob. The blob then moves right for the dwarf! As it does so, a tunnel is revealed were the tapestry lie.
Yet the blob seems confused. Forge is unengullfed (imagine on the above map a 10*10 blob just above forge).
Buckthorn, you open the other coffers. Between them you find 8 more pieces of platinum and 4 more gems, 1 moonstone, 1 piece of jade, and 2 pearls.