In spite of the constant impotent gnashing of teeth on here, and in the wider world, it's going to be impossible to prove that Mr Johnson did not act in good faith when following the advice he confirms he was given.
You can't mind-read and prove that someone did or did not know something at a specific time in the past.
All Mr Johnson has to do is continue to maintain his position - that he believed the information he was given to be correct, and he cannot be blamed for subsequent infomrtion proving it to be incorrect.
You can't try and condemn someone on the basis of 20/20 hindsight, that is not how the legal process operates.
The media, MP's, and the great and good will have to accept that it's not a matter of what they 'know', it's a matter of what they can prove, and that is clearly going to be impossible.
We will go through the motions, Mr Johnson will be condemned as a liar or a fool, he will not care one jot, and we can all return to normal.