What I do think is that anyone who pretended there wasn't a mistake, downplayed it when they knew it could be causing the prosecutions or even significant reconciliation issues, should be looked at.
First and foremost, in the UK, Fujitsu itself can (and I think should) be prosecuted for corporate manslaughter here if there is any evidence that Fujitsu executives conspired with the Post Office to keep this quiet, ever.
If anyone ever ordered a developer not to go public with their complaints, threatened a whistleblower, deleted evidence, that should crush Fujitsu for good.
But yes, individual executives should be at risk (the corporate veil can be pierced to include directors), and it's difficult to see how it is possible to rule out right now that there is real liability down the chain.
The situation at the Royal Mail is likely to be dramatically more serious, because there's a significant chance they were both covering up serious failings and knowingly directly prosecuting people to do so.