Simon 5 is coming soon!
Simon version 5 is currently in beta testing. It is a massive update, introducing a much-requested feature: syncing the Simon data between multiple Macs, plus a Simon Status service, a Link Checker filter, improved Find filter, iMessage and Speak Error notifiers, a new app icon, and much more.
Important: please note that Simon 5 requires a minimum of macOS 10.12 (Sierra), and will be a paid upgrade from Simon 4 after the beta. Purchasers since September 1, 2020 automatically get a version 5 license (that also works in previous versions).
Re: Removing Failures from History
There isn't any way to edit the statistics at present. It is something that is occasionally requested, so I'll consider it for some future release.
The only options currently are to reset the statistics, losing that history, or edit the plists directly, which isn't recommended (or much fun if you have lots).