All you are going to get are opinions, and they're all worth what you're paying for them.
1) It's not your responsibility to tell anyone anything. A simple "he is taking some time away from work for personal reasons" is all the explanation anyone is owed. If they press, you can defer them to him-- "he can tell you more if he wants to, when he returns". He can then choose to be as forthcoming as he wants, but what he'll learn over time is that he will intuitively know when it's the right time to broach the subject. He doesn't have to come out of rehab and yell "I'M AN ALCOHOLIC" like Michael Scott declaring bankruptcy. He should not feel obligated to "come clean" to anyone, until the appropriate time and place (making amends, helping someone by sharing his story).
2) Clearing out the beer was good. NA beer is not good. Stock it with Topo Chico or whatever.
3) Maybe and yes. Al-Anon is not really about helping recovering alcoholics. At least, not directly. It is about helping the Al-Anons break their codependence, which often times takes the form of enabling the addiction in the first place. Al-Anon teaches you (general you) how to accept that someone's alcoholism is not your fault, not your responsibility, and not something you can control, and how to detach lovingly. That indirectly helps the alcoholic by letting them experience consequences and understand that they and they alone are responsible for desiring and working towards sobriety.
4) That kind of depends. Right out of rehab, boozy get togethers probably aren't a good idea, but I don't know how revolted he'll be by the idea of relapse or of taking a drink. This is one thing you do need to address directly with him. If possible, he needs these to be optional until he's got some confidence in his sobriety and I do NOT mean the "I just finished 30 days of rehab and I'm free and I feel like a million bucks!!!!!!!" confidence, I mean he's worked the steps and has a real program in place.