Pull back wasn't in the original manual but was a relied upon method. Boeing fucked up years ago by not documenting the function so it was allowed to be dropped transparently. How is that defensible?
Why was a method that wasn't in the official checklist "relied upon"? Pilots following some undocumented, non-standard procedure sounds like their fault rather than Boeing's.
Who knows? Have you ever been taught an undocumented procedure by the expert and been told to use it regardless of what the manual states? Happens all the time? Is a pilot in a position to affect Boeing beauracracy?
By including features in the manual. The alternative is that they don't, people rely on the features, they change the features without notifying anyone, and when shit goes wrong they can say "It wasn't documented: shouldn't do that".
The real question here is how commonly this feature was used. If it was common, then not putting it in the manual is a big problem!