You're right that is a potential failure point, but I find it hard to believe that she didn't complain about it at home. It is also my understanding is that it is excepted wisdom that Akane's anti-boy attitude came about from the attacks and while Soun generally doesn't do unless it effects the joining of the Schools, Akane turning lesbian does actually fit the bill there. Beyond that there's the rumor mill, which should get things to him in time, I'm sure there's a canon incident of him showing up awfully quickly to 'remind' Ranma and/or Akane of the pledge.
I'm not saying it's impossible, but the level of negelct it requires is higher then normal for Ranma
Two new ideas:
1) Soun's wife died in a Martial Arts related incident. This could explain why he stopped teaching his daughters but for some reason not teaching in general. (Yes, I know it's weak, but you come up with an explanation for why he stopped teaching his daughters but not in general, otherwise you end up with them living off a trust or Nabiki the bread winner).
2) Shampoo feeds with a Love Potion Purgative somehow and they experience a reaction. Note that this is A reaction, they might not have been under the effect of one. (Try telling me Genma and Soun didn't feed on to at least Ranma)
PS: Where did you get the figure 3 weeks?