Previous Replies are Correct
Wife and I both have our own Delta Amex Platinum cards for the sake of the MQD waiver. I can confirm:
1. The designated companion gets no miles.
2. No free upgrade to C+ ever, but one can select exit rows in C-.
3. Not a true BOGO because taxes are still due for the companion.
4. If there are empty seats in first after all upgrades and all non-rev's clear into first *and* if the gate agent and the gods smile upon you, then you might be upgraded to first. (Happened once on SAN-LAX.)