This is certainly not the main entrance to the Palace, which is through the colonnaded portico between the two wings.
I suspect, though I have no evidence for it, that most people attending an audience with either the Tsar or the Empress, would enter through the main entrance and then be escorted through the rooms in the centre of the palace to emerge, on the ground plan, in the Small Library (2). If they were having an audience with the Empress, it would probably take place in her Formal Reception Room (1) or the Maple Room (70).
If the audience was with the Tsar, they would have gone from (2) into (1) then (69) then into the (68) the New Study.
Maybe only family members or regular visitors would use the entrance you are talking about. I say this because I suspect security officials would worry about the easy access into the heart of the Imperial accomodation that this latter door gives. Its just 3 doors to the Imperial bedroom..............
Anyway, I've just managed to buy Spiridovitch, so if anybody knew he would.......................