There's no plot hole with book 7 Fidelius Charm
So, I've complained in the past like others in the fandom that the fact Bill and Arthur can be their own Secret Keepers in Deathly Hallows introduces a new plot hole into how the Fidelius works. Because, if they could be their own Secret Keepers, why did James and Lily need Peter to be the Secret Keeper?
Well, I thought about it and I have an answer. It's not a plot hole, and actually, it works with everything we were told about the Fidelius Charm up to that point.
The Fidelius Charm is described as:
“An immensely complex spell,” he said squeakily, “involving the magical concealment of a secret inside a single, living soul. The information is hidden inside the chosen person, or Secret-Keeper, and is henceforth impossible to find — unless, of course, the Secret-Keeper chooses to divulge it. As long as the Secret-Keeper refused to speak, You-Know-Who could search the village where Lily and James were staying for years and never find them, not even if he had his nose pressed against their sitting room window!”
(PoA)
So, it keeps a secret, not necessarily a location, safe within the Secret Keeper. So, let's look at the different secrets the Fidelius Charm is used for in the books.
James & Lily
The Potters went into hiding about a month before Harry was born since that's when the prophecy was made:
“I did,” said Dumbledore. “On a cold, wet night sixteen years ago, in a room above the bar at the Hog’s Head Inn
(OotP)
Dumbledore says this on June 1996, a month before Harry turns sixteen, so the prophecy was made about a month before Harry was born. The date on the Prophecy in the DoM supports that.
In spidery writing was written a date of some sixteen years previously, and below that: S.P.T. to A.P.W.B.D.
(OotP)
So, when the charm was cast, it was just James and Lily in the house because Harry wasn't born yet. This is relevant because Hagrid says this:
No, sir — house was almost destroyed, but I got him out all right before the Muggles started swarmin’ around.
(PS)
If muggles can see the house when he arrives, it means the Fidelius Charm completely broke. This would only happen if the secret the Fidelius guards no longer existed. This leads me to believe the secret specifically referred to James and Lily as that's the only way it would stop working in this way upon their deaths. As such, I assume the secret Pettigrew guarded was phrased something like this:
James and Lily Potter are hiding in a cottage at X, Some Road, Godric's Hallow
So they were part of the secret and couldn't be their own Secret Keepers because of that. As I'll show in the other instances the charm is used.
Dumbledore and the Order Headquarters
Dumbledore is the Secret Keeper for Grimmauld Place, and, in this case, we are actually told the exact phrasing of the secret:
The narrow handwriting was vaguely familiar. It said: The headquarters of the Order of the Phoenix may he found at number twelve, Grimmauld Place, London.
(OotP)
So the secret is about the Order using Grimmauld Place as headquarters. This is why the Fidelius didn't break upon Dumbledore's death and honestly, Moody adding another defence on it kinda ensured the Fidelius would stick. He kept treating it like a headquarters, so the secret was still true and the charm would remain.
I imagine after the war is over, when the Order disbands, the charm on Grimmauld Place breaks because there is no longer an Order of the Phoenix to be the headquarters of.
Arthur and Bill
We also know the phrasing for the secret Bill is keeping in the books and I assume Arthur's is similar:
I am a werewolf, married to Nymphadora Tonks, and you, the Secret-Keeper of Shell Cottage, told me the address and bade me come in an emergency!
(DH)
It's not about who is at Shell Cottage, but about where She'll Cottage is. It's address, specifically.
Ron tells Harry and Hermione he stayed with Bill and Fluer in Shell Cottage:
Bill and Fleur’s new place. Shell Cottage. Bill’s always been decent to me.
(DH)
So clearly, the knowledge Bill and Fluer are at Shell Cottage and that Shell Cottage exists aren't part of the secret. Bill isn't part of the secret and, therefore, can be Shell Cottage's Secret Keeper.
I do want to note Ron is able to tell Dobby and others more or less where the cottage is:
“Bill and Fleur’s,” said Ron. “Shell Cottage on the outskirts of Tinworth!”
(DH)
The fact Ron can even say this means the secret is incredibly specific and not about Shell Cottage's location but the specific address (since he only gives a general location). Harry, when he apparates to it, even sees the cottage without Bull telling him where it is.
Basically, the secret of Shell Cottage is phrased so specifically phrased it doesn't really hide the house. Or anyone. This allows Bill to be his own Secret Keeper, but it also means they are much less protected and that someone could potentially locate them as Ron and Harry have done.
Honestly, it begs the question of why they bothered with the Fidelius at all, but I digress.
