You're right, those are problems. But they're not problems with this law, they're problems with jury-based criminal justice systems. And the system's had 900 years to evolve solutions -- judges summing up, "No case to answer" / directed verdicts, etc.
There's room for disagreement about how effective those solutions are. Point is, as the main objection to this law, that complaint 'proves too much' -- since essentially the same objections apply to all but the most trivial criminal offences, the solution then becomes scrap juries and go for a Germany-style professional/inquisitorial justice system. (And maybe we should, I don't know).
(btw, this is very, very far from being the most complicated law or subject matter that juries have to face, compared to e.g. complex fraud trials)
(To be clear, I think this is a terrible law! But just because I agree with TFA's conclusions doesn't justify his fearmongering, IMO)
It is not uncommon for juries to accept flimsy and circumstantial arguments no matter how hard the defense tries to explain that the Prosecutor is spewing nonsense. The scenario the author lays out is not certain, but it is plausible.