These weren't random faults or glitches. They were bugs in a system that had been developed very poorly, compounded by poor training, a helpline that regularly gave subpostmasters faced with unexplained shortfalls advice that actually resulted in doubling the shortfall and so on. A bug that gives an error in favour of Post Office will always give errors in favour of Post Office.
There were undoubtedly some errors in favour of subpostmasters. We know of some cases. However, the evidence is that the vast majority of errors were in Post Office's favour. We can see this from the fact that Post Office were putting millions of pounds each year into a suspense account because they couldn't figure out who the money belonged to. The money would sit there for three years then, if they still hadn't identified who the money should go to, Post Office took it as profit. It seems that no-one joined the dots and thought that this money might be related to the alleged shortfalls for which they were pursuing subpostmasters.
Paula Vennells became CEO when Post Office was spun off from Royal Mail. The CEO of Royal Mail from 2003 until shortly before Post Office was spun off was Adam Crozier, who has given evidence to the inquiry. Whilst Post Office was part of Royal Mail it had an MD rather than a CEO. All three holders of that post whilst Horizon was in operation (David Mills, Alan Cook and David Smith) have given evidence to the inquiry. Vennells left Post Office in 2019, at which time it was clear that the subpostmasters were going to win the group litigation, and was succeeded by Nick Read, the current CEO, who isn't implicated in the scandal at all. His involvement is in the compensation schemes.
Why is Vennells the focus? Evidence of Horizon's unreliability didn't start to emerge until shortly before she became CEO. She was in charge throughout the time Post Office was desperately trying to cover up the scandal. She was in charge when Post Office decided to try and win against Bates & Co by running them out of money. She was in charge when Second Sight were sacked for getting too close to the truth. She was the CEO who made promises to MPs and then broke them.
Fujitsu employees and ex-employees have given evidence at the inquiry. One witness still to come is Gareth Jenkins, the former chief architect at Fujitsu, who appears to have perjured himself in giving evidence that helped to get subpostmasters wrongly convicted. It is not yet clear to what extent his behaviour was due to instructions from Jarnail Singh, Post Office's Head of Criminal Law (a post for which he was totally unsuited based on his evidence to the inquiry). Those instructions were highly inappropriate for an expert witness and effectively told him to perjure himself.
My understanding is that the main people telling subpostmasters that they were the only one having problems with Horizon were Post Office's investigators rather than the helpline (which was manned by Fujitsu employees). We don't know, but I suspect this was driven by John Scott who was Head of Security at Post Office until 2016. This is the man who ordered that documents related to Horizon should be shredded to prevent their disclosure. This is the man who saw subpostmasters as enemies of Post Office. This is the man who failed to mention that he had been a policeman in his witness statement, leading to Jason Beer, counsel for the enquiry, asking, "Did the documents that we sent you assist you to remember whether or not you were a police officer?"