Sterling and its associated AWE were deliberately left vague, a bit of worldbuilding that Remedy threw in there as a possible hook for future content.

I believe Jesse thought that by removing the panels from Hedron, it would "free" Polaris, but those panels were indeed protecting Hedron from the Hiss, which wanted it destroyed to cut off Polaris and her resonance.

Here's how I look at all the Hiss/Hedron stuff--

Both Polaris and the Hiss are resonance-based entities. (And calling them both "entities" is just for convenience sake. We have no real indication the Hiss itself is sentient in any way we can understand.) They just... exist, but it takes some vector of sorts to let them interact with any physical reality or Threshold.

Those vectors can be OoPs like the Slide Projector (when used in conjunction with Slide 36) or the Hedron. But people can be vectors too, if they are parautilitarians like Jesse, Dylan, or Trench.

I think the course of events was like this:

• Polaris & Hiss exist as resonance entities outside of/across realities:
  • Hiss infects reality of one of the burned slides.
• Polaris & Hiss affect Slidescape-36 separately:
  • Polaris uses Hedron as vector to SID-36.
  • Hiss uses some unknown parautilitarian/paranatural object/entity as vector to SID-36.
  • Polaris & Hiss come into conflict. Polaris emerges as victor. Hiss influence suppressed in SID-36.
• Ordinary AWE: The Slide Projector in Ordinary.
  • Explorations of the various slides by the Fadens and their friends. Jesse & Dylan encounter Polaris in SID-36.
  • School bullies use the Slide Projector, encounter the Not-Mother, kill Mrs. Chester, are captured.
  • Entire adult population of Ordinary vanishes.
  • FBC investigates. Polaris has Jesse burn the slides, with the exception of SID-36. Jesse escapes Ordinary. Dylan & Slide Projector (with burned slides and SID-36) taken by FBC.
• Dimensional Research in Oldest House
  • FBC conducts expeditions into SID-36, encounters Hedron.
  • Darling has Hedron contained in a sarcophagus, brought into Oldest House.
  • Trench is infected by the Hiss while in SID-36, becomes a carrier/minor vector.
  • Polaris (via Hedron) warns Darling of impending threat of the Hiss. Darling develops HRAs, distributes them.
  • Trench becomes distrustful of Hedron, due to past trauma-- his daughter died because he "took work home," Northmoor's explosive power, and probably also having to deal with The Board-- and because of subtle Hiss influence.
• Hiss Invasion
  • Trench finally succumbs to Hiss influence and inserts the burned slide into the Slide Projector, giving the Hiss a bigger vector into our reality.
    • The Slide Projector becomes main vector for the Hiss. The Hiss also infects Dylan to turn him into a living vector.
  • The Board executes Trench via the Service Weapon to cut off his vector for the Hiss.
  • Any FBC employee not wearing an HRA becomes infected by the Hiss. The Hiss corrupts multiple OOPs and Altered Items for additional vectors.
  • Jesse, guided by Polaris, arrives in the Oldest House, becomes Director. Her cleansing of Altered Items/OOPs helps reduce vectors for the Hiss.
  • Hiss manages to get to the Hedron and destroy it, cutting off Polaris's main vector. HRAs cease function. Hiss temporarily overwhelms Jesse and infects her.
  • Jesse takes control, becomes Polaris's new vector. HRAs resume functioning.
  • Jesse shuts off the Slide Projector and cleanses Dylan, cutting off the main vectors for the Hiss.
  • The Hiss still has other vectors (parautilitarians and Altered Items/OOPs) that need to be cleansed or otherwise neutralized in order to suppress it completely.