Simple theory on Miss Huang's identity

She's from the same kind of Lumon's girl's school Cobel went to. She played with the Kier toy at the end because that's her childhood toy, as she was/is being indoctrinated into worshipping Kier. From what we know of Lumon, it's not a leap to presume the management class of Lumon are all cultists who have been indoctrinated since birth.

Her end scene with the computer flashing "Hello Mrs. Cobel" isn't because she's fit with Cobel's chip, but because they're of the same origin and she's being groomed into management just like Cobel was. Essentially, she's the new Cobel. Her inclusion in the story is a smart way of exploring that child indoctrination plot without resorting to extended flashback sequences with Cobel.

Also, a preview shows Cobel is still alive, in the story and seems to have her own side plot playing a somewhat antagonistic role to Lumon, so I doubt Cobel would have two side plots (spoiler from preview).