This is horribly poor customer service. It sounds like you did everything by the book. My suggestions:
1. Send written correspondence again, return receipt requested, to Richard Andersen's office (I hate to say it, but don't expect a return phone call - at best expect a snail-mail response). You might also try sending the correspondence in some form of odd-size/shaped packaging. Maybe a box or something. Something that will stick out and be somewhat inconvenient.
2. There is also a fax number that you can find on here. I'd fax copies of the correspondence to that fax number

In your correspondence, be very clear about what you're looking for Delta to do to resolve the problem, but don't try "making threats" such as "I'll begin flying Pan Am if you don't do this". I suspect that making threats will just put them into a defensive posture and they'll be less likely to try to take action for you.

My own personal belief is that Delta should, as a gesture of goodwill, bite the bullet and make your friend GM, even if it turns out that you were given incorrect information multiple times. It's a one-off exception and would help alleviate a bad relationship with a SkyMiles member that they theoretically should value.

Please let us know what response you get.