Hi Gleb!
It seems to me the first floorplan shows the Palace at some earlier time (no doubt someone will know at what time). Hau's watercolor was done in 1861 (it's signed and dated), indeed after Alexandra Feododrovna's death (1860).
Looking closely at this watercolor, it's obvious to me a very narrow room can be seen, then the Empress' s study (corner room). Therefore, her room must have been No. 30 on the second plan, not No. 29.