The alternator test is always accurate, it just depends if the problem is constant or intermittent. If that checked out, they should have performed a battery test as well since the alternator didn't seem to be the cause.
So far things still seem to point to a control device (ignition switch or module). The ignition switches are a more common problem on Grand Prix's. But that still doesn't explain the battery being dead as long as it is still in good condition...
Oh and I just re-read the post, and saw that you mentioned that you couldn't lock the doors when they're open. There is an "anti-lockout" feature that I know is on the drivers door, that will not let you use the lock button while it's open. Mine also dings when I do that.
