I find it interesting that you mentioned your anger toward hubby, difficult child 2, friends and LASTLY difficult child 1. When a person becomes 'angry with just about everyone' - I'd say that's a symptomatic of something - ya know? Really - it might be a good idea to talk to your doctor.
I think your husbands letter was very balanced and very sensitive. I think he makes sound arguments in terms of your difficult child. It is obvious he still very much loves your son - but also recognizes the futility of 'coerced rehab'. You can't force someone to want to get better. I wish we could - but we can't.
Detachment really is necessary - if you want to help your son and stay sane in the process.
I'm really sorry.