hey... horrifying thought... Romeo never actually died before becoming the king of puppets
end of the DLC there's a post credits scene that has Geppetto show up with the nameless puppet box, and while we don't see it crawl out we do see a shadow walking toward Lea before cutting to black and we know Geppetto uses the nameless puppet to fight for him, so it fighting and killing Lea isn't a far stretch.
but Romeo was there, what happened to him?
he's only barely consious, he doesn't even open his eyes, he had a forced double amputation + massive prosthetic attachment done in probably the last few hours, he's not really waking up for a good few days, maybe even a week or two, and even then there's infection to worry about, not just normal bacteria but also the petrification disease... the boy could easily lose a good two-three months to minimal recovery alone. That's not counting physio and PTSD management
but we do know that geppetto worked with the alchemist, and the alchemist were capable of removing a brain and keeping it alive and aware, there are diagrams all over the place and even in his office of the human brain and skull. so... possibly, geppetto takes romeo, and either chips away at his body because of the petrification disease (real or just a lie) or he takes romeo's brain and puts it in a puppet, claiming to have saved him from bleeding out at the Rose estate... He wouldn't have to convince Romeo to fight the alchemist or stop the disease, he already knows what both are capable of. But also now he's hooked into every puppet in Krat, can communicate with them indirectly, gets a notice from the train station that someone's wandering around, not looking sick but everyone else is (or they're dead) and eventually he learns what this person looks like and what train car he walked out of and realizes that those vague half remembered bits and pieces weren't his mind playing tricks on him, geppetto was there after Arlecchino, Lea did really yell at him and fight him or a puppet or something.
he probably has a rough understanding how Ergo and Ego work, He knows the rules all puppets must follow... so he figures (at least some of it) out.
but he also knows Carlo, knows that if Carlo's ego awoke in a puppet capable of killing, of lying, of independence, he wouldn't listen to a damn thing geppetto had to say... but if that Ego was awoken in a puppet WITH the Grand Covenant implemented, Carlo wouldn't be able to say no
notes in the DLC imply that an ego awakening can be helped along if the puppet looks like the person, so Geppetto made P (because he's not Carlo, not anymore) look just a bit different to stave off a full awakening until he could get the arm of god.
also, in the second phase romeo says he's burning up, now I know I saw a post before the DLC that figured maybe Geppetto didn't install fan's in Romeo's man body as a means of control, keep him in the big King costume that can't leave the theater (it's got a giant fan attack guys) and maybe that along with an ego awakened made him feel the burning... like it was psychosomatic. but what if there are still some sensory nerves still there and still firing?