4. timestamped record of every change you have ever made in htaccess
But that's proving to yourself. Do you need something that's out of your control, so you can't ever have tampered with the documentation? (Obviously any idiot can fiddle with the timestamp on a local file, digital photograph or similar. But the person at the other end may not know this.*)
Do you need to prove that the redirect was not in place at all, ever? Or only that at some specified time, a request for A did not point the user to B?
Proving a negative is a pain at the best of times.
* I have the impression that printed copies of email are accepted as evidence in at least some courts. This is obviously insane.
The 404 was not much help. It is not very complete.
I did find the indexed report.
As expected it shows a huge drop in pages indexed during that time period. Would it look exactly the same if all the pages had been redirected. I assume that a redirected page would not count as a "page indexed" on this report.