Eberron: Rising from the Last War

If you want to head back to the Statue Room to give it a go i'd say there's a chance it could work.

Alright, Erix will head back to the statue room and have a look to see if he can find a way for him to enter the treasure room as a spider or ant.

Perception (I presume): 19

And what are the others doing? If Erix goes back to the statue room you would all see him. So you could tag along, or you could continue searching guest rooms.

Might as well give the last two rooms a try, except listen at the door before opening.

Mark will look for the others. Who would he see? 

“Or stand back, let’s open the door from afar, wait 30 seconds and see if anyone comes out.” As kit says open the door from afar a spectral mage hand/ tangle of tentacles crawls over his shoulders and begins drifting towards one of the unopened doors. “Say ready.” 

Elero will pick whichever door Kit is not aiming at, and listen at it.

As soon as Mark walks out of the common room he would see Elero and Kit standing in the hallway next to him. 

As Kit opens the other door with the Mage Hand and waits as Elero makes a DC 10 Perception check to listen in on any conversation behind the last door: 16

Elero hears two voices talking openly, a man and a woman, by the sound of their voices they seem noble in stature. The woman admits to the man, "I feel sorry for Alden, I really do. The man is clearly still in love with Kwanti d'Orien, and in her absence is only the boy, Caden. What was the name of Kwanti's train again, something about a horse was it?" The man responds, "The train is called The Silver Unicorn. She spends all her time travelling the world looking for new House Orien projects, while Alden stays here looking after her heir. To be honest it doesn't seem like a bad gig for the man, except for the broken heart and all." The woman giggles, clearly amused by the man's aloofness, "Oh Taris, you would have loved to stay home with our son wouldn't you? I think despite that, he knows you love him. I wonder if he is coming tonight?"

Kit waits expectantly outside the door the Mage Hand just opened. After a few moments, a woman pokes her head out, a half-elf with glasses and her hair up in a bun. She first looks at Kit and frowns, "Did you open this door? Why?" Elero hears the voice of his dean at the university, Kalrey d'Medani. She is currently absorbed in questioning Kit, but if she turns around she will see Elero.

Elero will quickly turn away from her and walk down the hallway until he's out of sight of her. "People in both rooms," he'll Message to Mark.

Mark nods and will say, "We are wasting time, we need to get a move on or this whole operation is blown. Round the others"

The half-elf woman will see Mark talking behind Kit, "And who are you? What is this? This is very strange, i'm going to have to have a talk with Vishtai I think. I would not expect to be disturbed while getting ready for a ball. Not in a place like this."  She goes to walk back towards the ballroom.

If neither Kit or Mark do or say anything she'll walk back towards the ballroom to the west.

Erix get's to the area in the statue room marked as having a secret door. As he looks closely, he notices that the wall in that part of the room has a very very thin crack that outlines the shape of a door. There is even a small round hole in the stoneface in the place where a key-lock would usually be.

Can Elero see if he can turn the All-Purpose Tool into a shape that'd work in that key lock?

Mark will bow his head low immediately and say aloud, "Apologies Madam, we were sent to make sure that you needed anything in preparation for the event tonight. We did not mean to be a bother. Please do not waste your time on a pair of wreches like ourselves. We will leave you post-haste and make a note to the rest of the staff that you shall not be bothered, Mrs d'Medani."

He will also hint to Kit to also bow. 

Erix will have a look if light comes through the key hole, meaning it goes all the way through so he might be able to get in by wild shape. 

Erix - Yes there is light that comes through. A kind of strange purple light.

Mark Deception check with advantage with Kit bowing as well: 20

The half-elf squints at you for a moment, then relaxes. "Well if that's the case, I wish for some warm water. I need to completely redo my make-up as i've ruined it now." She smiles and adds, "Sorry for the misunderstanding. I'm really not used to these kinds of events." She goes back inside her room and closes the door.

Elero - From having read the All-Purpose Tool write up, it can only turn into a kind of Artisan's Tools, so unless you can think of a way that will help, I don't think you can make it turn into a key for a specific lock.

Erix will turn into a spider and crawl through the lock to see the treasure room. Telling Elero "watch the lock, if I come out as spider and not turn back to myself I got the item and we should leave the premises immediately."