Also, Cooper didn’t even know how to lower the aft stairs.
Again, let's invert your statement: "What does Cooper's not-knowing how to lower the aft stairs tell us?"
Here are the quick answers I can think of:
1. DB was a dumb ass.
2. DB was suicidal who didn't give a s**t.
3. DB was too chicken to jump off the Tacoma Narrows Bridge like most normally depressive people when they get really suicidal. (See the bridge jumping debate at the DZ in 2010 that got the cops and CPS in Florida involved. Georger and I were personally involved in this, as I remember. I talked with the mom, G talked with LE, as I recall.)
4. DB was too lazy to read that section of the operations manual in Blg 9-10 at Boeing.
5. His mommy forgot to tell him about the stairs.
6. DB had the hots for Tina and needed an excuse to keep her on board. (This is my personal favorite.)
7. DB never had to open the stairs before because someone else had always done that when he had jumped previously at Moses Lake, Laos, Cambodia and North Vietnam.
8. DB was more of an "Ideas Guy," and not so much an "Hands-On-Details-Guy."