oop! just gonna squeeze right past ya!

one problem with a theatrical adaption of tlt is htn, where the reveal that Gideon lives on works because of the change of second person to first.

the only way i can think of it working is that the actor playing gideon works backstage, like the lights system (but is hidden from the audience aside from subtle hints)

the biggest hint is when when wake breaches pal's river bubble she 'breaks' the lighting system and the stage goes dark. harrow is ushered into the wings by pal so she doesn't see anything, but the lights flick back on just before the curtains drop for a scene change, and pal looks directly up at the light box in surprise and smiles. if the audience is quick to turn around they can see a flash of a black robe.

Oh boy my friend, have you come to the right place!!

So, fun fact about ninja. Bear with me, I am going somewhere with this. The image of a ninja covered head to toe in black, with a hood and mask, comes from Kabuki theatre. It was originally a stagehand uniform. Like stagehands in modern theatre, stagehands in Kabuki would wear all black to signify that they were not really there, and whatever effect they were causing (carrying a prop, creating a breeze, ect.) was to be taken as happening on its own. Basic stagehand stuff, a lot of productions in many styles around the world do it, especially if they don't have fancy rigging systems.

Someone (I don't remember who now, or in what play) had the idea to dress the ninja in a production up as a stagehand. In the convention of the theatre, this made them invisible. The audience was already so used to ignoring stagehands, they didn't know any more than the characters that the ninja was present, despite the actor being clearly visible on stage. Which meant when the ninja struck, it was as if out of nowhere. I can only imagine the uproar in the theatre the first time it happened. It worked so well as to become commonplace, and the rest is history. The popular image of a ninja is still a kabuki stagehand.

So, back to the stage play of Harrow the Ninth. I think you've hit almost exactly on how to incorporate the Gideon twist into a theatrical production. But not as a lighting tech. Gideon is a stage hand. Maybe there would be more than one stagehand, maybe she would be the only one, but she would operate in full view of the audience, literally setting the scenes. I think it works best if she's the only one, but if the production needs more, she should subtly stand out in some way. As the play went on, we would notice that this one stage hand... increasingly interacts with Harrow, though Harrow never acknowledges it. At first it might look like she's playing Harrow's necromancy, because that would be the main special effect she would need to help with. When Harrow is unconscious at the end of a scene, it's always the same stagehand carrying her out. But we all know she's not really there. Until Palamedes acknowledges her. Turns to look right at her, and speaks to her. I can see the scene clearly. He would look at her, stunned, until Gideon finally took off her mask. The line "Kill us twice, shame on God," would be addressed to Gideon, and then he would turn back to Harrow, kiss her on the forehead, and tell her to go. Gideon, always out of Harrow's line of sight, would guide Harrow away while Harrow looked back at Palamedes.