> If (say) 50% of convictions "turns on whether you had a lawyer ahead of time to tell you what not to say", is the law ridiculous?
Yes.
It's entirely believable that at least 50% of laws are ridiculous.
> for cases where it's particularly easy to feign ignorance/innocence, sometimes the law specifically states that the mens rea requirement is waived (i.e. the intent/knowledge does not matter, or does not need to be proved beyond reasonable doubt).
But that's not the issue.
If you go out to the parking lot and your key fob unlocks someone else's car of the same make and model, e.g. because the manufacturer screwed up and made any of the keys open any of the cars, and then you claim that you thought that car was yours when you drove off in it, the prosecution now has to prove that you intended to steal someone else's car. Which they may yet even be able to do, if there is other evidence that implies you knew what you were doing, but it's completely reasonable that they have to prove it.
Whereas if you went out into the parking lot and broke the window of a car and then hot wired it, your actions are evidence of your intent. And, importantly, it's generally difficult to avoid actions that signal your intent while perpetrating a car theft.
The actual problem for this type of securities laws isn't just that the prosecution has to prove your intent, it's that their only means to do that is effectively a confession. There is no plausible way to operate a chop shop such that hiring a lawyer ahead of time would keep you out of prison when you get found out, but for parts of the financial industry that's what happens, which is absurd. There is no justice in a law that can only be enforced against people without effective counsel.