give the incorrect answer that she knows.
Why do you keep saying this? In the fiction, Eloelle is a genius. She knows the truth. But at the table, because there is "5" written in the INT box on the character sheet, the GM is not telling Eloelle's player anything useful. And in the fiction, the reason that Eloelle spouts this ignorance and nonsense is not because she believes it, but because she is directed by her patron to do so.
if it would give the PCs access to knowledge greater than her 5 int, then it isn't the Zone of Truth breaking the game, it's the character concept itself.
My view is that if a single corner-case spell causes problems or complexities in an otherwise interesting and workable PC concept, it is the spell rather than the character concept that should yield.
I don't know 4e, but if it compels the truth, then there is no basis for an "I don't know" instead of a check. The check would have to be made and an answer given if the check succeeds.
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How does that narration collide in your game? All it does is give a reason for his success with knowledge skills. A PC with a +40 and no such narration would have identical knowledge. Being forced to tell the truth is working as intended in 4e, with or without that narration.
I feel that you are not really engaging with the question that I have raised.
I'll try again.
The character in my 4e game has very little chance of failing knowledge checks. Hence, as a practical matter, the player of that character can acquire pretty much whatever information he wants. In the fiction, this is the PC's recollection of some experience from one of a thousand past lives.
However, the player (and hence PC) doesn't actually know everything, because only a finite number of knowledge checks will be declared and resolved in a given session.
In other words, there is almost nothing the character
can't know, but there is plenty that the character
doesn't know.
Under a compulsion spell (ZoT or anything similar), is the player obliged to make a knowledge check to hand over information (and hence, in practice, be an unlimited encyclopedia for the enemy magic user) or is he allowed to declare "I don't know"?
If you take the mechanics as a literal model of the fiction, the answer would seem to be the first. I don't take the mechanics to be that, though - the mechanics are a device for working out what happens at the table when the player wants to resolve a knowledge check; they are not an exhaustive model of what, in the fiction, the character knows.
As I said, it has some features similar to Eloelle - the interaction between the narrative and the default approach to the mechanics (in this case, the knowledge skill mechanics) can produce some quirky outcomes, but there are other ways to make sense of what is going on which help keep things on an even keel, and don't drive a big wedge between what information the player has gameplay access to, and what information he can be forced to yield up under magical interrogation.