So we know that ogLucia is still inside her body. But what about ogAlexandra?

She is the "og alexandra"?, she just regained her past memories. In which is why she changed

With that logic, is Lucia and ogLucia also the same person?

Yes ?

??? Wasn't it supposed to be two souls inside Lucia? OgLucia and the reincarnator. It was already mentioned in the manga.

???? I think i misread lucia with Alexandra but what?????

basically lucia reincarnated/transmigrated into oglucia's body, whereas alexandra remembered memories of her past life, so ogalexandra is the same as alexandra as she only gained memories rather than have a new person go into the body. i hope that made sense

Different souls, same body. With OGLucia turning out to be a yandere of sorts for the new lucia.
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OGLucia basically helped orchestrate the fall of the CP and Lucia so that she'd get Lucia all to herself.

It's more like she's "thankful" to "Lucia" because when "Lucia" explained to Oglucia that they're in a world of a game Oglucia became aware that they're going in a loop where she makes the male leads fall for over and over, So she orchestrated oglucia's fall so she could break out of the cycle. (Someone explained it in the comments, they read the novel)