I agree keep letting your lawyer handle things, an listen to his advice. As far as what he sent you be sure to pass that to the lawyer, since he admits to using cocaine in his letter that might help if it does come to a custody battle.
Your husband may very well have stopped using for now until things settle down. But if that's the only reason he'll start right back up again soon as he thinks the coast is clear. In the letter he is trying to justify his drug use in my opinion that doesn't sound like someone that's going to stay clean.
You've held up pretty good so far so just stay strong take it one day at a time an let the lawyer do his job.