Just to put my two cents in on the assassination idea: I have to disagree with the idea that assassination is somehow so outrageous an idea that Section 31 would never call for it. First, what basis do we have that S31 is "gentler" in the 22nd century than in the 24th? As far as we know, we have only met two agents (four if you count Trip and Reed). Do we really know that Phuong and Harris have never killed anyone? Do we know that they have a firm line that they do not cross, where they don't assassinate leaders of dubious allies? I have seen no evidence that Harris or Phuong or any of the presumably many other agents wouldn't be perfectly willing to kill someone. And a trend of two is not much of a trend.
To me, the idea of assassinating T'Pau is fully within the thinking of Section 31, as I have no reason to think they are not as ruthless as their 24th century form. And in the 24th century, they assassinated the Federation President! Not an ally who they disagreed with, but their own leader, and not to prevent something from happening, but to cover something up. It's totally up their alley to think, "Oh, T'Pau is keeping needed assistance away? Well, let's be rid of her, then." There could be plenty of reasons for their not assassinating her, such as deciding it would not help (if the Vulcan government is the parliamentary one people are assuming), or too risky (that Vulcan might discover the truth) or even they know T'Pau's true motives (beyond obsessive pacifism).
Also, there is a false equivalency here. Just because a reader believes that it makes sense for a character to do something does not mean that the reader is advocating it. If I say that it was likely for the Punisher to gun down Archie when he mistook the kid for a criminal, does that mean I think it's ok to gun down criminals? No. It's within the character of the Punisher to do so. Not me. I think it is unfair to cast aspersions on Mach5 for that. If anything, maybe he is more capable of thinking outside of his own perspective. I didn't even consider the possibility of assassinating T'Pau, but once it was said, it made sense that some might consider it an option.
And finally, yes, we all know T'Pau is alive in "Amok Time." We also know that Earth will survive, the Romulan/Vulcan connection will not be discovered at large for another 100 years, Trip survives til 2238, and despite all likelihood at this point, the Federation is founded. Mach5 was not calling for the assassination, he was wondering if there could be an attempt, knowing full well that it failed.