This is something Im pretty familiar with after using almost every class of psychoactives in an attempt to get off of heroin. For me, Diazepam was helpful (in fact quite necessary), but it was limited in scope. Their main benefit in this situation is in the ability to help one sleep, which (depending on the seriousness of the withdrawal) can become near impossible. Diazepam is personally my favorite as well, with xanax being a close second....things like clonazepam generally did nothing for me. But while having them is a HUGE ally in coping with withdrawal, it didnt seem to help me whatsoever during the day. Mundane tasks like getting up to make food, shower, etc (and worst of all...going to work) become battles with depression, lethargy, pain, etc. For me Benzos just didnt seem to have the ability to alleviate these symptoms in any noticeable manner. For that, I needed to look to other substances to get me through the day. After several years of H addiction, what really helped me to quit was a regimen of stimulants during the day (to give me the physical strength/energy to still function in my daily life) and Benzos at night to be able to sleep. This was then halted shortly after to keep from developing another addiction. I realize this is a harm-reduction site so I can't really recommend that method...but it did do an amazing job for me.
As far as alcohol, that's an even more interesting issue. In his book "Junky", the elder statesman of opiate use sir William Burroughs stated, "When you use junk you don’t drink. Seemingly, the body that has a quantity of junk in its cells will not absorb alcohol. The liquor stays in the stomach, slowly building up nausea, discomfort, and dizziness, and there is no kick." Ive personally found this to be the case as well. During an opiate addiction or during acute withdrawal, alcohol use is often uncomfortable and by the end of the night usually turns dysphoric. On the other hand, after the acute withdrawal stage is over alcohol can be an *extremely* helpful tool. After the more intense acute symptoms began to subside, I still suffered from physical pain, depression, and severe cravings for months afterward. Alcohol was really a life-saver in helping me to pull through the after-effects of this situation. Pragmatically it does work well as a pain killer and the alcohol euphoria will start to come back after a bit...so it does help immensely both mentally and physically. I've only known one opiate addict personally who still enjoyed drinking during full addiction all the way into the withdrawal stage, and alcohol helped him...so it might be possible, but it depends on how bad the symptoms are and how long you've been using. But my generally opinion is that alcohol is a poor choice during acute withdrawal, but can be a very valuable ally directly afterwards.