For your 757 leg that is an emergency exit. The door protrudes to where the 3rd seat would be. You cannot store anything in or around that area. Treat it as if it was a bulkhead until after takeoff.

That being said 20F uses that area for legroom.

There is not much legroom on the 900 as there is a bulkhead in front of you. C and D in row 10 is nice because you can stretch out into FC.