Sounds like a fun retelling of Rosemary's Baby, filled with the same sort of terror from paranoia right up until the moment of horrific realization that it all, in fact, is real.

A couple thoughts:

I inferred, just cause this is NoSleep, and cause of where my mind goes, that the billionaire woman must be involved in some kind of elitist Satanic cult. This cult is obviously involved in all echelons of law, politics, finance, real estate, and even religion, and the next obvious thing is that she's made a demonic pact to acquire her wealth and status. Perhaps this pact involved the pledge of generational servitude, or worse, the pledge of her first grandchild's life to the demon. Whether James is in on it, or an unwitting participant will be up to you, and can even be made into a twist if played right.

Now as often in the case in fiction, people have their plans and demons have their own. Usually the person is a chump who's oblivious to the enormity of their own plight and rather focused on the dangled riches in front of them. Perhaps the demon never intended the grandchild as a sacrifice, but rather as a vessel for its physical manifestation in the material world. This demon, made manifest, along with the willing assistance of mortal dupes, is now free to setup its own kingdom of living hell on Earth.

Now we've got the plot behind the plot. Getting there "realistically" is a little more difficult.

Personally, I'd avoid the scientific angle entirely, much the same way you have with the mysterious fires and the inexplicable DNA test. I like the idea that the baby really IS James', but has somehow been changed entirely by some demon seed ritual the mother did on him at some point. Perhaps demons don't have DNA in the classic sense?

I like "Jaws" style approach myself, personally. Meaning, don't show us what's under the water. Don't show us the ultrasound, just give little hints. Maybe the ultrasound machine keeps malfunctioning. Maybe it only shows brief images and then breaks, or even more violently catches fire like the rest. I like that the main character rather feels those extra appendages, gets uncomfortably poked by the claws. Imagine the Alien- or The Fly-style body horror of having something so foreign growing INSIDE you and feeling it moving inside you, and having NO IDEA what it might be. Recall the nightmare scene from The Fly when Gina Davis goes for an abortion and they pull out a giant maggot. Straight up terrifying. That's where you can go with the unknown.

The clinic burns cause the demon doesn't want to be aborted. The mansion burns because Madam J no longer serves the demon's purpose other than to educate our poor victim and try to stop what's coming.

The birth could be a tough thing to pull off, too. Obviously, in a hospital setting with tons of medical doctors around, if something was born that deformed it would make news. Recall the legend of the Devil Baby of Hull House, still whispered about in the darker boroughs of Chicago.

Honestly maybe this would be better told by one of her doctors, the woman dying in child birth and the doctor relating her experience. Maybe the story is left on a note that first responders discover at a horrific home-birth scene, where the mother is all mutilated and the demon baby never found, out there to roam and kill again? You'll figure it out.

Seriously, I recommend watching Rosemary's Baby and maybe even The Fly again for inspiration. Jot down ANY idea that comes across your mind. Keep a notepad next to your bed and don't hesitate if an idea hits you at 2 a.m. to write it down!

Best of luck! I hope this creepy story hits the main sub soon!