No 4 is the killer.

Deficits are paid back by the postmasters and branch owners to the tune of millions which went where?

In bonuses. To the bosses and those 'investigating'.

And those law-abiding postmasters were so ashamed they paid all the deficits and made themselves bankrupt and poor.

The Post Office preyed on the entrepreneurial middle class business owners and their fear of being shamed in the community. So they kept quiet. Until a few were brave enough to fight for the truth. But that has its cost as well.