I think the best preventative measure for next time would be to try to only hire directors who a board member can personally vouch for- they've seen, or better yet, been in their work.  It also helps tremendously if you hire directors from "inside," ie, people who have a longstanding commitment to your company, even if in another capacity than directing.  But, unfortunately, sometimes directors just don't work out, and no amount of Monday-morning quarterbacking will eliminate the possibility.  At this point, as others have suggested, find someone responsible who knows the show to get in there and oversee the rest of rehearsals.  Good actors tend to be able to survive directing fiascos.  If they quit, that's another matter.

As far as two other preventative suggestions, just my comments:
1. on having potential directors AD a show- this has limited value.  Directors tend to be very cautious in this type of situation, and you're not going to see what they really have to offer- and how they do it.
2. I think having a lot of oversight of a director during rehearsals can be problematic.  All an overzealous board or producer does to a director is compromise his ability to create.  It will also raise tensions.  I've been in situations where I knew what I was doing, but the board didn't trust me, and it just made my job that much harder.

One thing that does work is promoting a friendly environment, so that the director will trust you if he needs someone to go to for assistance.  That way, any potential conflicts can be nipped in the bud.  Make a point to the director from the getgo that you trust his abilities, and give him some breathing room.  That way, he will be more likely to ask for help when necessary without being afraid that his show is being stolen from his hands.