343i is currently going through a storm of issues with Halo 4. The only thing that can be done at the moment is to wait for a fix. Bringing this up on the Waypoint forums could also help, but I'm sure there have been tons of reports about this.
Its very hard to believe 343i did not notice these issues while beta testing internally. The achievement glitches are very common and I suspect 343i turned a blind eye in order to ship the DLC in time.