Where can I get a list of SCHEMA / PATH / KEY to use with gsettings?

gsettings list-schemas gets you all the schema. You can also use gsettings list-recursively for what you want but this program will list all the values for all the keys for all schemas:
(Lest's call the script gsettings-iterate-all)

#!/bin/bash
# Gnome 3 can be customised from the command line via the gsettings command
# This script should help you to find what you're looking for by
# listing the ranges for all keys for each schema

for schema in $(gsettings list-schemas | sort)
do
    for key in $(gsettings list-keys $schema | sort)
    do
        value="$(gsettings range $schema $key | tr "\n" " ")"
        echo "$schema :: $key :: $value"
    done
done

Expanding on your example gsettings-iterate-all | grep com.canonical.Unity2d.Launcher yields

com.canonical.Unity2d.Launcher :: edge-decayrate :: type i 
com.canonical.Unity2d.Launcher :: edge-overcome-pressure :: type i 
com.canonical.Unity2d.Launcher :: edge-responsiveness :: type d 
com.canonical.Unity2d.Launcher :: edge-reveal-pressure :: type i 
com.canonical.Unity2d.Launcher :: edge-stop-velocity :: type i 
com.canonical.Unity2d.Launcher :: hide-mode :: type i 
com.canonical.Unity2d.Launcher :: only-one-launcher :: type b 
com.canonical.Unity2d.Launcher :: reveal-mode :: type i 
com.canonical.Unity2d.Launcher :: super-key-enable :: type b 

You can reroute the output to a file for easy reading.

And for creative folks out there. Here is a list of possible options to gsettings that might help create other scripts.

It's a bit late, but I just started wrestling with this privacy concern...

It looks like com.canonical.Unity.Lenses remote-content-search 'none' is the toggle you seek.