This has been brought up a few times. Teldrassil has been cleansed of all corruption since Cata. The finale quest of the entire zone is using the power of Elune to root out the remaining bit of Old God corruption left by Xavius. Afterwards, it’s blessed by Alexstrasza and Ysera, protecting it from relapse.
Malfurion, if not Druids in general, are sensitive to that kind of thing, with Malfurion sensing something wrong in the Dream well before the Nightmare has played its hand. It’s why Fandral had to keep him comatose to grow Teldrassil in the first place. If there was an Old God gnawing at the roots of it, he’d know. He’d certainly know long before an undead derivative Elf from the other side of the world.
If you’re meaning N’zoth, we have seen him in-game, as of the Eternal Palace raid. His prison was somewhere near the location of Azshara’s Eternal Palace, nowhere near Teldrassil. “Was” being the operative word. Spoilers:
Helya has in all likelihood switched sides, and at the very least, works for the Old Gods indirectly (as if being all tentacley wasn’t a big enough tip off). She was swayed to Loken’s side, after Loken’s corruption by Yogg Saron, and helped Loken further Yogg’s escape plan by sealing Odyn away in the Halls of Valor, just as she had done the Elemental Lords in the Elemental Planes.
Well, firstly, as seen in the cinematic, his prison was not below Teldrassil. Secondly, proximity is not required for N’zoth to attempt to take control of someone. Spoilers again: But part of the Wrathion questline on the PTR is N’zoth trying to exert control over Ebonhorn/Ebyssian, who’s located all the way within the Chamber of the Heart.
No.
A few have tried to desperately grasp at these straws before. The theory has been found wanting.
Not that it would be the the first pants-on-head story direction of BfA, if they did try to pull this off.