I wouldn't be so quick to blame law enforcement for inaction. First of all, if she did drive (or was driven) off the brow, chances are she died on impact or shortly thereafter. Nothing that law enforcement or anyone else could have done (then or after) would bring her back from the dead.
Furthermore, there were several citizen organized search which also failed to discover either the jeep or her remains. What do you say to that?
There's not enough evidence processed at this point to either prove or disprove the husband's and/or girlfriend's involvement, much less rule suicide or negligence in or out. At least law enforcement has tangible evidence and a scene to process now. If there's anything to be found it will reveal itself soon enough. After 7 months, a few more weeks of forensic investigation isn't going to alter the outcome.