I think that only Traitors and Highwaymen are the only things you boil in oil, but you have the right idea!
Wear some heavy gloves and eye safety googles when messing with the hot oil. put some cooking oil or lard in a large pot and heat-up to between 325-240 degrees.__ Cut your horn as you described and with tongs, hold the horn in the hot oil for only a few seconds at a time and take out of oil, and continue dipping your horn in and out until the horn seems to be getting pliable.
Now set the horn down on a piece of wood or ? and wedge a screw driver in the cut of the horn and with your gloved hands. Slowly pull the horn open. You might have to repeat heating the horn several times, before you can get the horn piece forced open to put between two pieces of flat boards and start pressing flat in a vise. Like I said, you probably will have to re-heat the horn piece several times to get your horn totally flattened out! ____ The secret to flatting horn is to keep it hot and press it slow! Now, just let it cool overnight in the vise jaws with boards and it will be ready to sand flat and use tomorrow!
Rick