I'm one of the moderators on DIYAudio, so some of this has been discussed on our site too.

My personal opinion is that groups should be fully open to public viewing with the restrictions that you listed newaag.  It's always a difficult balance between providing a public service and encouraging technical content that is sometimes guarded information.  Certain manufacturers have had me sign NDA's before they would release any information to me - and I was a warranty service shop! So blocking the view and / or downloading  of content would be expected and enforced.

With new members, we moderate their posts until such time that we have a good idea whether they will be disruptive or not.  Bots and spam oriented members are banned before they are released in the vast majority of cases.

I have seen some super private sites where you only discover they exist after you are formally invited.  In audio, this means that you're going to be dealing with engineers and therefore need a certain level of understanding in that field.  The main agreement beyond good behavior is that you don't even talk about the site.  So for people who are thinking along those lines, a new sub-group could be created with those privacy options in place.  No one would even know that the group existed and therefore all posts would be private outside of that group.

Editing posts.  I believe strongly that you should only have a set time where you can edit your post.  Say, 1/2 an hour.  You should never be allowed to "change history" as it can put following posts out of context.  Our moderating team will not edit a post (we can) except to remove personal information.  Excepting special circumstances, we copy and past the post and send it to the poster with an explanation of what the problem is, then we delete the post in it's entirety.  Moderator actions are reviewed by others on the team to keep our actions objective.  It's sometimes a very difficult path to follow when there are more than one correct action.

-Chris