[minsc_tdp]

Keep in mind that the structure taking on this pressure is at the bottom (see the sea floor?) So it's a consistent 1000 LBs of pressure, it doesn't change. Also consider that the structure is very tiny, so the bottom chambers are, like, 1001 LBs of pressure at most due to the additional depth. This is a hypothetical scenario, keep in mind.

Regardless of the extra 1 LB of pressure, if Answer #2 is correct, it is due to the fact that there is only 1 or 2 LBs of pressure once the first chamber is full, due to the fact that the first seal still has 999 LBs of strength, so the 2nd seal only has to hold back 1 or 2 LBs of water with 999 LBs of strength which is no problem. Remember, the first seal is never fully destroyed, just a slow leak.