- Why wasn't a fourth Oath required, like "I will never seek to serve the Dark One or willingly work to advance his goals."?
A quick refresher on the Three Oaths (mostly for my benefit since I couldn't remember the second oath):
- Speak no word that is untrue
- Make no weapon for one man to kill another
- not use the one power except against Darkfriends/Shadowspawn or in the defense of her life, that of her warder, or another Aes Sedai.
You're right in saying that a fourth oath might have made sense, but it's important to note that the Three Oaths were put in place immediately following (or perhaps during) the Breaking of the World. The Oaths are intended to get people to trust the Aes Sedai again -- after the male Aes Sedai destroyed most of the world, forced truthfulness guarantees trustworthiness (or was supposed to), and the other two Oaths guarantee that they are "safe" to be around. Re-establishing trust with (and power over) the rest of the world was critically important.
Since the Breaking began with Lews Therin's sealing of the Dark One, such an oath may have been deemed unnecessary, but that is pure conjecture.
Finally, such an Oath could possibly have been added later, but by that point the Tower's culture might have solidified to the point where such an Oath would be considered an insult (see my answer to your second question below) or perhaps was prevented by the Black Ajah's machinations.
- Why weren't newly raised Aes Sedai questioned immediately after taking the Oaths?
As others have mentioned, the Aes Sedai suppressed all information about the Black Ajah. Even inside the Tower, mentioning the Black Ajah was unthinkable, in no small part due to the Oaths.
When
Talene
is revealed to be a Darkfriend, she is indignant about the mere suggestion, which seems to be a typical reaction:
[Seaine takes the Rod, swears the Oath, and declares she is not a Darkfriend]
"This is ridiculous," Talene said. "There is no Black Ajah."
[Seaine, Pevara, and Yukiri take the oath and hand the Rod to Doesine]
Talene frowned in disgust. "Stand aside, Doesine. I for one will not put up with this filthy suggestion."
[Doesine offers the Rod to Talene]
The golden-haired woman started back as from a poisonous snake. "Even to ask this is a slander. Worse than slander!" Path of Daggers, chapter 26
Except that the others had already taken the oath, and she was caught.
Some members of the Black Ajah, like Liandrin, sought out the Black Ajah from the start of their time in the Tower (The Fires of Heaven, chapter 18), but even they swore the Oaths and later had them replaced:
"Let the world know her. Let people bend knee as had been promised when she first forswore old oaths for new." The Shadow Rising, Chapter 28. (emphasis added)
Indeed, even Galina swore on the Oath Rod first:
"[Galina] had been broken free of the Three Oaths on joining the Black Ajah, replacing them with a new trinity ..." A Crown of Swords, chapter 40
And she knows that an Oath Rod can unbind as well as bind, else she wouldn't be so desperate to get it from Therava.
So every Black sister seem swears on the Oath Rod first, then swears new Oaths later.
Alas, the most surprising member of the Black Ajah,
Verin,
mentions nothing of this ceremony, only that it is "...distinctive." so some of the oaths are probably not on the Oath Rod, although we know that the Oath Rod can counteract them.
- Could the Oath against lying not be easily circumvented by just making various statements while a novice or Accepted? Then, just preface everything with the words "I said." Seems to me that a person under the Oaths could give a false answer that way. "I said yes." (eighty years ago that one time while I was a novice).
Yep, that seems like a pretty easy way to get around it. Additionally, some of the Aes Sedai hypothesize that
Mesaana,
who is in the Tower, could use a simple weave to make it sound as though she was saying the Oaths even though she was saying something else entirely.
