How should I convert my range receptacle?
What you think is incorrect is exactly what I'd do and what I have done in the past. Ideal make a blue wire nut that would work for a #6 connected to a #12, Do that for one hot leg on both ends and the one end of the neutral where the outlet will go. Install a 15 Amp breaker and connect the #12 pigtail to it. connect the two pigtails at the box to your new outlet. Don't forget to connect the ground wire. Cover the box with the appropriate cover. You could also use split bolt connectors instead of wire nuts. Just make sure you have about a mile of electrical tape. Tape up the ends of the unused wire.