I don't think that PWM is available when using the companion certificate. I tried to book a companion reservation and couldn't get the PWM option to come up. I did copy this from the terms & conditions of the certificate: "Primary Ticket and Companion ticket must be purchased with your Delta SkyMiles Credit Card from American Express". This seems to imply that the only form of payment eligible for the booking in it's entirety must be a DL SM AMEX.