First of all, this is a beautifully crafted theory, and I love it, and I think that these all could work as oaths.
That being said, I am a Truthwatcher myself (identified by both the test and reading the order description), and I feel like I need to put my own two cents in.
I noticed a pattern in the way the descriptions of the Orders are set up: For the orders we know oaths of, the beginning of their second Oath is the subtitle of their description. (off-topic ramble: I think that we should call the First Oath the "Radiant Oath" as that is where Radiance starts, and the next oath the First Oath of the specific order.) Windrunners start with "I will protect those who cannot protect themselves", Skybreakers with "I will seek justice above all else", and so on.
Therefore, I believe that a Truthwatcher's second Oath will be something along the lines of "I will seek truth instead of lies" or "I will seek the truth and share what I find."
If you take that as a base for the next oaths, the third, going off your oultine, would be: "I will seek truth not for my own gain, but for the good of all"
The fourth as you wrote it does not fit completely into this framework. Your explanation of the fourth seems spot on, so a better oath, and fitting with what the fourth seems to be for a Windrunner, would be: "I will not force others to accept the truth."
Your fifth fits in almost perfectly, and I think would be necessary for all Truthwatchers. However, I think with the focus on seeking the ultimate truth, the final thought about truth isn't cynicism, as you say. Yes, it is important to keep hope up, but the act of pursuing truth does not destroy it. I think the final oath would be "I will accept when I am wrong"
Anyway you spin it, this theory was great, even if your oaths seem a little more Edgedancer to me. I'm fully prepared to admit that I'm wrong.