To try to answer your questions... Provided his brain is now fully active phisical opiate withdrawl should set in pretty quick. The doctor may have meant that since he has been out he may have to deal with a lot of the psycological issues that go with addiction. However, as weird as this sounds, he is probably dealing with so much other pain and feelings, the mental aspect of the opiate withdrawls may never enter into the picture.
He had been clean for two weeks before the night he ODed. His withdrawals had to have been almost completely over at that point. He most likely decided to use because he was/is psychologically addicted and it probably had nothing to do with easing withdrawals at that point.
And as much time has passed now, his withdrawals are LONG over.
Many people kick an opiate addiction and go back after the withdrawals are over. No matter how badly you destroyed your life, no matter how horrible the withdrawals are, it's easy to go back. You become comfortable living like an addict. As messed up as things are, you always know that all you have to do is shoot up and everything will be fine for a couple of hours. It's instant relief when you can't handle life.
Before I got seriously addicted to heroin about a year and a half ago, for the first time in my life I had everything going for me. I had a wonderful girlfriend that I would have ended up getting married to, I had a good job, I had money, I had direction and I threw ALL of it away. I had to start over again. And even though I know I could work to get everything I lost back, it's a lot easier to say "well, I've already lost it all, at this point I've got nothing to lose" and just go back lying to myself saying "I'll just quit again before I get sucked in all the way". It has nothing to do with withdrawals.
As for the Ativan, I'm absolutely sure that Dave knows it's a benzo and not an opiate. There may be a conscious decision on his part that he wants to put something in his body, but not any opiates because of what happened to him. He probably is under an extreme amount of stress and anxiety and could benefit from some Ativan as long as his use was being monitored by a nurse or doctor.
Ativan is NOT a painkiller. It is an anti-anxiety agent. It does, however, get you high, especially at higher doses when abused. Although at normal doses, it's not as if he's going to be noticeably high or anything. It would just calm him down. There is the risk that it may feed his addictive personality, maybe that's why he wants it. Who knows? But it's not like if Dave gets high on Ativan that he can go out and start abusing drugs again.