I had recalled reading something about Rudy similar to this from the 2025 Dragoncon transcript.

"If Harry successfully killed Nic, would Rudolph be a good host for Anduriel? No. Nobody likes Rudolph. Even the Fallen are like 'That guy? Give me something better to work with'. Nic is a main series villain, Harry will be dealing with him till the end, take that for what you will. " [From the Transcript on Reddit].

Also just for everyone's amusement, when I asked Google to search for this transcript the A.I. gave me this:
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In the Dresden Files book Battle Ground, the character Rudolph is unable to pick up a Denarian coin because he has lost his free will and soul due to psychic manipulation, making him an unsuitable host for a Fallen angel.

Here is a breakdown of the events and the reasoning:
The Incident: During the battle against Ethniu, a Denarian known as Blood on His Soul (who manifests as a bear demon) is defeated. After its host is killed, the Denarian coin is left on the ground.

Rudolph's Attempt: Detective Rudolph tries to pick up the coin, likely tempted by the power it offers, but he is unable to grab it.

The Reason: The coins of the Fallen Angels of the Order of the Blackened Denarius can only be willingly taken up by a mortal with a functioning free will and soul. Rudolph had previously been a victim of psychic manipulation by a Nightmare (Kravos) and later by the agents of the Red Court (Esmeralda and Esteban Batiste), which left his psyche broken and him without free will.

Implications: Because Rudolph was essentially a puppet and lacked the free will required to make the choice to accept the coin, the coin "rejected" him. This demonstrated the extent of his mental and spiritual damage and confirmed that the coins require a genuine choice and soul to inhabit a host.
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This wasn't in the copy of Battleground that I read...

Curious from the bones lying around if the foe is

The Gatekeeper never actually went onto Demonreach during the series. He very specifically never set foot on it cause Alfred held a grudge against him. He went somewhere close by the Island but still on the lake.

There was also the path that Peabody used to get near Demonreach in Turncoat that was an alley or something in Chicago that Mouse and the P.I. Kid staked out.

The show writers, directors, and producers do need to be faithful to the series as much as possible.  However having said that viewers/fans have to realize what not everything on page translates on to film very well and have to be changed.  I think if Jim takes the time to work closely with the script writers it will be okay..

I don't disagree about what you saying about changing the color of an actor, but that doesn't bother me half as much as changing the sex of a character.. Because the character remains the same, just a different actor of color plays him or her.  Opera has done it for years, Tosca remains Tosca no matter the race of the singer playing her.  Dr Who is the latest, suddenly both the Doctor and the Master are women?  Why?  There are strong Time Ladies out there, Ramoda was one of them, so why change the sex of the Doctor? They did it with Captain Marvel as well.. Okay, now I wonder when Wonder Woman will become Wonder Man or Black Widow becomes Black Widower?

The bolded part will only happen in Disney buys out Warner Brothers. Wonder Woman is part of the DC Comicbook Universe while Wonder Man is part of The Marvel Comicbook Universe. I think you are safe from that for awhile.

And I still don't know who told Harry all that about the pieces but I want to know what all of them (including the Grial) do in the Dresdenverse.

Off topic but I do wonder why the Real Shroud of Turin hasn't been used to either heal Helen Bickett's daughter (aka Persephone) or Murphy. Although honestly it is Persephone that has me wondering more cause Murphy likely declined if offered.

I'm still suspicious of Erlking, in the sense that I think he's got a second identity we've seen. 

I'm not sure what trouble Margaret would have caused the Erlking.  It might be interesting to consider what she would have been doing that would impact him.

The Erlking did say that Harry being LeFay's child made Harry's "visit" to his domain make more sense.

Said visit was in Changes when Harry & Susan were being chased by the Ebs, their Red Court Vampire entourage, and the Ik'k'oux.

Margaret Lefay likely dropped in unexpectedly a few times with some antagonistic +1's.

They killed his daughter and they are evil monsters. Do you need more reasons?

They killed someone else important to Eb. Someone that ranked higher in his thoughts than his daughter. When he mentioned that they had taken someone from him, Harry mentioned Margaret and Eb said "Her Too."