This was taken from an article I read about the film. Not from Rian himself, but I think it speaks to this discussion:
"Knives Out hints at the self-righteous Thrombeys’ racist views of Latino people and immigrants in general well before they target Marta and threaten to have her mother detained. In the film’s early flashbacks, the Thrombeys each referred to Marta’s country of origin as a different place. To one, she was a lovely young woman from Ecuador; to another, Marta hailed from Paraguay. Their cluelessness is played for comedic relief as the film’s tension builds, and Johnson’s message is clear: The Thrombeys’ ignorance is the joke, not Marta’s heritage."
Toward the frenetic ending, Detective Blanc discovers which Thrombey relative really conspired to kill Harlan. Marta’s morphine mistake is shown to have been a setup: Someone else had switched the medicine labels, meaning Marta hadn’t erred on the job. As ever, she’d been paying attention to Harlan, and to his needs. The revelation is at once tragic—Harlan didn’t have to die in order to save her—and liberating. No longer fearing a future of incarceration, Marta is finally able to relish the one thing the Thrombeys most want to deny her: peace.
Maybe we're looking at this all the wrong way.
Maybe Marta's MC problem has more to do with the Future... what will happen to her/her family. I'm thinking of those scenes when she comes back home, looking at her mother. You can almost hear her thoughts, "Oh mom, what is going to happen to you? To us?!" It seems she's the only one concerned with that in the film.
Even Ransom, who actually committed the crime, isn't concerned with the future. He's concerned with getting what he's owed.
Permit me to entertain this:
MC Marta: Universe, Future -- Will I get away with the coverup/Will I and my family be deported?
IC Harlan/Ransom: Mind, Subconscious: All that talk about Harlan subconsciously treating his family the way he did, what he wished he could do instead. Ransom being rejected by a particular group. Desiring the inheritance for himself, or just desiring to do things for himself on his own. In for a penny in for a pound.
And here's the twist... I can almost see OS and RS switched in this case.
We made the assumption that the OS is in Psychology, but could all that Psychology stuff really be RS? And the Obtaining (catching the bad guy, busting someone, acquiring something, etc) be the OS?
**Sidenote: This arrangement would also put this storyform in the same place as Rian's previous film, The Last Jedi. Same mind telling another story. His version of a mystery--which did feel pretty fresh/unique to me.
Also, from Subtext. Mystery Trends: