Notice that stair symbols on the map have a "narrow" and a "wide" end. The "wide" end is the high side, the "narrow" is the low side. The stairs in area 6 are thus descending; on the middle level map, the stairs nearest to area 15 ascend, so they are the same stairs as on the upper level. The other stairs, nearest to area 17, descend away from the middle level, so they are the same stairs as on the lower level connecting to 22.
All three maps are in the same orientation, which should help.
Yes, as I explained to Cryanhall above (his question #2). I've updated the Entry to include a brief blurb about this situation, but if you started with the original entry, you don't need to switch to the new one, just read my answer to him.
From second the level entrance (stairs on bottom right) =>
Down 3 flights of stairs; Door to room 15; down 3 more stairs reach door to room 16
*** Then go down 3 more stair flights to "long corridor on Left" ***
No, 16 and 21 are not on the same "level" in terms of altitude. 18-21 are at the same level, 90 feet down from ground level. 15 is 30 feet down, 16 is 60 feet down, so there's a 30 foot drop from 16 to 21. 17 is 150 feet down from ground level, or 60 feet down from 18-21, and that drop is explained in the text blurb (though now I think I made an error, the 5' pit should be 40' down to a 10' cubical room, not 50' as I stated before; the total drop into that 10' room is 50').
I'm uploading a new entry that fixes the drop error from 18 to 21.