In a few months, mortgage insurance giant Radian Group will close on its $305 million cash purchase of Clayton Holdings, ending the “independent” status of one of the nation’s largest MBS due-diligence firms. Almost all the larger due-diligence companies have been gobbled up by larger players over the past 18 months. Most of the acquirers have other interests in the residential finance industry and are betting on the eventual return of the non-agency MBS market. That could be...