Nardole, while dressed as a monk, brings the Doctor River Song's diary. In many ways, that book is a bible for the Doctor—it's full of meaning for him and tells him how to go about right and wrong. It's even literally about someone who died to save him.
The Doctor was escorted throughout the flashback; from the moment he arrived on the boat to the moment he pulled the lever, he was in sight of the Fatality Index Guy. So when the heck did he "fiddle with the wiring"? Well, when FIG is explaining how the execution equipment works, the Doctor replies, "I know how it works." The only time he could have sabotaged the device is before his official arrival. Which means he was going to spare Missy all along! (This also would explain how he ended up in the position of executioner — he never meant to carry it out!)
Why is the Doctor here, not only as an executioner, but also taking on the obligation to guard the Vault for a thousand years. This goes completely against character — what sort of hold do these experts in death have over the Doctor, that he would allow himself to be in such a position?
The Doctor, according to Nardole, came here from Darillium. Were they the reason he had to at last leave River Song?
This is the second time the Doctor sacrifices something of his future self to achive his goals. Back then it's possible the Doctor hadn't yet decided that he didn't wish to regenerate again, but here it's almost certain that he was creating a debt he had no intention of paying.
