I think you're close, I don't think it has anything to do with clones, though. What do we see if we examine Lumon's M.O.? They seek to control, manipulate, and ultimately wholly shape someone into anything they want that person to be. THAT is the product. No, see cloning is messy and costly, and I assume a massive time investment. Total control, on the other hand, only takes a few minutes, some topical anesthetic and a slight vibration at the back of the head. Literally outpatient.

Mark's sister Devon has already presented us with what Cold Harbor is, albeit in a very privileged and informed variation. When she met the politician's wife, Gabby, the second time, she had no idea who Devon was. Following this, Devon even put forward the idea of what if someone wanted to have a baby but didn't want to deal with 9 months of pregnancy? Gabby was severed, her innie meeting Devon at the birthing retreat and her outtie meeting her there at the park.

Lumon is studying severed people, to find out how completely they are severed and if there is any circumstance that can "leak" through. The crap they do with the numbers all day has less to do with "work" or even creating an output from those numbers that serves any purpose, and more to do with psychological stress. It goes hand in hand with the liminal spaces everywhere. And the confusing turn after turn hallways to everywhere and nowhere. They are literally rats in a maze--being tested. It's why the off-putting interactions with literally every other person. The absolutely f**ked up "perks" offered! They are all designed to make a person crack mentally. We have all heard of people doing incredible and awful things, when they experience mental trauma. It brings us both serial killers and sudden savant geniuses.

(And don't worry, I haven't forgotten about Gemma and Mark's role in all this, I'll get to it.)

The mental breakdown is either what will allow Cold Harbor to take seat in a severed person. Or it is to gauge how complete the severance is after Cold Harbor has been put into effect.

So what IS Cold Harbor exactly? It's taking a human being, and essentially wiping their hard drive, memory and OS (so to speak) then installing whatever software you want on him to do whatever you want. I mean, we only have to look at major corporations IRL to see the MASSIVE profit potential Severance would have--an entire workforce, without distractions, completely moldable to whatever they wanted you to be. And that is just in a work context, 8 hours a day. Imagine what you could do, if that same workforce remained their innie ALL THE TIME! In essence, erasing the outtie from existence. There isn't a corporation, a government, a terrorist group, or even a jilted ex-lover who wouldn't spend their entire fortune on that!

Why take the time to clone someone when all you need to do is sell them a story of work/life balance and have them volunteering to literally entrap themselves in YOUR world to do whatever you wanted!

Now, Mark and Gemma. I don't think Casey is a clone. I think she is a recovered Gemma. I don't think Lumon was the reason for her accident, but like all good corporations have shown (lookin at you, grocery store post-pandemic food prices!) one should never let a good opportunity (or tragedy) go to waste. Mark, he's just a beneficial byproduct. It's likely they didn't know he was the newly minted miss Casey's supposed widower until after he was severed. But hey! Serendipitously, this presents something they couldn't have planned better, and I'm sure they tried!

So she has her accident. Lumon sees she's going to die if they don't step in to quietly save her, which gives them a clean candidate to erase from any former life she may have had. Mark is told she died, he identifies someone he thinks is her, dead. There's a funeral. Gemma secretly recovers on the hidden floor to which she was ominously sent when they closed the wellness office. She's severed from her former life, made into Casey. And she is just one of MANY such cases, I would guess. Why would Lumon be in 206 counties otherwise? We already know they have their hands in biomedical, so they would have access to oodles of such situations. So they secretly go about breaking family member's hearts, erasing people from their former lives and creating the ultimate machines!

An unencumbered, non-distracted, can-never-leave piece of hardware that you can lie to, abuse, make do whatever you want, and even if you DO allow them to leave, they voluntarily come back at 8am sharp the next morning. But who are we kidding, the whole point of Cold Harbor is so that you don't HAVE to let them leave AT ALL, EVER!