... Did we ever conform Papa's shield is Outsider-powered? It's a good possibility, maybe probability ...
I *think* so. I think Harry got an "Outsider-flavor" from his attempt to cast on Raith Pere.
... the likes of the Forest People have an immunity, so maybe someone else can bestow it too.
They aren't "immune" -- they simply understand the magic well enough to ground a spell and dissipate it. Presumably enough power, or enough different spells cast simultaneously, could overwhelm them; or if they were unconscious, they couldn't do it.
Papa Raith just has an aura that sucks the magic away, whether or not he "does" anything. I suspect Harry -- with the right approach -- could have Starborn'ed his way past that aura, if he had understood in the earlier books what he understands in the later books.
His children were acting against him in secret ...
He had largely controlled his daughters. They weren't acting
very much against him, because he could have had any secret out of them in a trice. He had killed all his sons except Thomas, who only survived because:
- he assiduously avoided Whamp Politics, building a power-base, or anything else that Papa could have construed as a threat
- (I have a WAG that) Margaret may have hinted to Papa that she had set "special" protections on Thomas, and Papa was unwilling to risk it after she had blocked his ability to Feed his Hunger
In general, though, "acting against <X> in secret" is, I think, just the water Whamps swim in... always plotting, always being plotted-against; always conspiring, always being conspired-against.
... he probably told no one about the protection granted to him by The Outsiders, Lara probably suspected ...
Over the (many) years, I expect Raith had been hit with magic more than once, within sight of other Whamps (particularly his daughters (who seemed to act as personal bodyguards?)). That he had
some sort of protection seems unlikely to be a well-kept secret. The details, though... I suspect he intentionally dropped multiple contradictory hints. Make a maze of dead-end clues, swim in a school of red herrings. Show up as having a unique & potent ability, that nobody understands.
... Lara probably suspected, making her ripe for approach, perhaps by one of the former Archives. All she would have to do is to be aware of the Archives role and to write “My father, Lord Raith is in contact with The Outsiders, Lara” and then destroy it ...
Lord Raith wasn't AFAIK any kind of "native" talent. All his magic was cast by following written instructions. The Archive already knows all the stuff in his library, Lara doesn't have to tell him.
My own WAG is that most of the Venatori cells in the Oblivion War don't know the Archive is their ultimate boss, nor the mechanism of the Archive in recording information. It would paint too big a target on the Archive, for an Outsider attack. As noted, Lara's will wasn't entirely her own. Papa could have had any of Lara's secrets, and I suspect the Archives wouldn't have risked knowledge of their role & methods in such close proximity to a ruthless mindbender like Lord Raith.
Instead, I posit that the Archives give out "special books" -- heavily-enchanted journals, wrapped in spells of obfuscation & misdirection, with an absolutely tremendous KABOOM booby-trap at the core. They tell their contacts that whatever is written in these journals, will also be written in a matching journal held by the "Oblivion Council." This hides the fact that the Archive sees
everything written, and creates a plausible mechanism for 1-way notices.
