"The true power of the founding titan can only be used if it resides in someone from the royal bloodline." That's what I said.. also, the fact that one with royal blood + founding titan would eventually be consumed by the thoughts of the 145th king doesn't really mean much.
Not eventually, this happens as soon as you acquire the Coordinate/Founding Titan.
This seems to be a gradual yet inevitable outcome, but up until the point they allegedly decide to take their own life they would be in control of the full coordinate abilities. They can still access that power fully up until their demise, the kings will wouldn't prevent them from accessing the coordinate in any way.
Yes, it WOULD prevent them from using it, I'm sorry, but you're wrong, and your whole statement contradicts itself.
If people were in fact able to use the Coordinate at first (for a while), then... why hasn't anyone ever used it?
They made a big deal out of this during the Uprising arc, and recently (with the revelation chapters) it was only further confirmed.
When Frieda was eaten by Grisha, she had been a shifter for only
3 years. Why didn't she use her powers then, if this was a "gradual process"?
Because she was under the influence of the Will, as is clearly depicted here:
When asked to do something about it, Frieda struggles, closes her eyes, and when she opens them again they are clearly
different. Why?
Because she's under the influence of the First King's Will, that won't allow her to help/fight back.
Even
before the encounter with Grisha (as stated by Historia) she had moments where she'd go batshit-insane:
Why? Again, look at the eyes. Clearly under the influence of the Will.
If this wasn't "accurate", if this was a "gradual process" as you're claiming it to be, then explain to me this: why does Frieda become influenced by the First King's Will
from the very instant she becomes a shifter, as seen here?
Why did Uri become influenced by the Will in the very same way,
the very instant he stepped out of the Titan's body after consuming his father? Again, look at the identical eyes (and Rod's realization).
Wanna go even further back? Sure, here's Uri's and Rod's father:
"[...] My father acted the same way
when he inherited the King's ideology."
Not "after a while", not "after a few years", but
when he acquired it. It's something that kicks in instantaneously if you're a royal.
These chapters were pretty self-explanatory, and the recent revelations only served to further confirm this. As seen in the first page that I posted before, if you tried to use the powers (Frieda holding her head), you'd be driven to suicide before you could do anything. That's why Frieda was crying, because the Will
physically prevented her from doing anything, and there was nothing she could do about it.
So yeah, I know this is SnK and all, and theories are fine, but at this point in the story this isn't something that's even up for debate.
If
any person with royal blood inherited the Founding Titan (Historia, Zeke) they wouldn't be able to use the Coordinate because of the Vow that the King made, period.
If, however, Historia were to eat ANY OTHER shifter, she would be able to do the same thing that Zeke does, and be able to control SOME Titans, but thats it.