Hell fire didn't happen until after Harry realized that Sheila was the Shadow. There is no evidence that the Id had gotten it going before Harry became aware.
Harry's
Fuego was smelling of brimstone (without his conscious intent, nor awareness of Sheila as the Shadow) -- and Harry was stressing on it, understanding it to be an influence of Lasciel. We don't know, with any certainty, that it was providing any substantive power-up... or that it
wasn't. But it was really there; it wasn't just the Shadow playing mental games. Will smelled it: it was part of recounting how Harry had become darker, scarier... and more-powerful. The fight with Cowl
scared the Alphas; they saw for the first time that Harry was moving to a power-scale that was simply out of their league.
I call that "evidence" ... I don't call it "proof!" But I'm advancing a
theory, not demonstrating a
fact.
When Harry went after Rudy, we saw:
Out of control rage
Muderous intent, and
acting on that intent.
Smell of brimstone.
And
all the actve Knights of the Cross -- the people tasked with redeeming the Hosts of the Fallen -- taking the field to contain Harry.
The theory fits. Other theories fit, too. This one fits... better than most. (YMMV)
... We know while he was talking, but the point is the Id has never been able to take over Harry...
I'll argue that it has, a (very) few times. Id!Harry
almost "took him over" outside Victor Sells' lakehouse, before the climactic battle (it showed us that Harry's dark-side and will-to-power were very strong, sometimes only barely leashed); and a few other times (e.g. when Molly provoked him to slag the garbage-can outside Priority Health(name?) aka the New Velvet Room. It managed to fully do so in Battle for Chicago, when Rudy killed Karrin.
Mostly, the id's urging Harry are just that: "urges" (often dark urges, like most of us have). Mostly, Harry is VERY good about keeping them contained. But nobody's perfect. And while Id!Harry almost never gets to pick exactly what Harry does, there is
some influence on Harry's choices & actions.
... This didn't happen, so it is natural that the Shadow would try a different angle, Harry's Id. The Id has always been Harry's dark side, long before Lasciel came along, but needed because a lot of Harry's strength comes from it. Another Classic Star Trek episode, that Jim has borrowed from, a transporter accident where Kirk is split into the two side of his personality, the good conscious side and his Id if you will or the dark side.. Turns out he needed both to be what he is and survive/
100% agree with all of this.
Thing is, the "Sheila Gambit" didn't work, and then "Lash" didn't work either -- in fact,
Lash turned against Lasciel!
So the Shadow retreated, to try again. This time, working longer with Id!Harry... staying even
further "in the shadows." Not even giving him the brimstone... until revealed by Angelic action.