The party thief is in possession of the locket. They know it is important, but nothing else. (as per Keravath's post, we've got it but don't know how to use it - yes I have scars from this encounter)

The paladin is the first in line in a 5' wide passage/stairs 30' from the landing and animated armor (landing/armor are at the top, party at the bottom). The barbarian/BM is behind the paladin so it takes 10' of movement to move through his space, then 20', ending up 10' short of the Armor. To reach the armor on the first round would require action dash, then action surge to attack/shove, assuming BA used to rage, leaving 2 attempts, if shove succeeds, then they can open the door and move into the room. Armor has 17 ST, no Ath prof so raging Barb w/ Ath has hi prob to succeed, but will trigger an OpAtt as he steps to/through door. Then on Armor's turn step back into blocking position and attack.

The first obstacle of this encounter is the 5' wide passage/stairs just to get to the landing and armor so that you can attempt to move it out of the way to access the door. The landing is 10x10. The passage/stairs are 5' wide, 30' long.

Your party does not know that the Armor has any vulnerabilities or immunities. You can only learn by trying and observing.

DM ruling - yes you can ready an attack action (1) and use that action to attempt a shove when the trigger condition is met. You can only shove a creature into an unoccupied space and 5' or prone.

There is no party bard. Any attempt to persuade the Armor that it should betray it's allies will have a VERY high DC.

The BBEG legendary action to blow a kiss has a 30' range. The KISS is a spell attack but it does not list if it includes verbal components. We will assume somatic only. Reminder 3 of the party are at DISADV to all saves. BBEG will cast Evard's and Cloud Kill on first two rounds (passing conc to witches) unless someone forces something different.

As written a party of 6 lvl 9 PCs = a Very Strong party and it is recommended that the DM Add another Barovian witch and a thug. Give Esmae another 30 hit points and increase her AC by 1. It takes Esmae 8 rounds to complete the ritual. Our DM nerfed this recommendation some.