I took it that the realtor was speculating about the fiancee as in those with money marry looks. I haven't seen anything that really confirms there ever was a fiancee other than heresay. With respect to the fiancee, has there been any confirmed notices of the engagement? These are fairly common and more so amongst those with money. I'm thinking something along the lines of an engagement part, upcoming wedding announcement or stag & doe gathering. Also, if a wedding was upcoming, the future bride may have started making plans like booking a hall, the church, etc.
One option to this is his gf is Informant #1, informing police who the suspect was with the "Ambition" tattoo, and who also knew of activities at the farm where "something" was buried... I'm sure only a select few would have known about that, and maybe even the gf wasn't told of something being buried at the farm, but had seen evidence of digging there. After their engagement was called off, for what ever reason, gf was bitter and now is getting her revenge. (Perhaps DM wasn't as loyal as she expected...or was more involved in activities she disapproved of... could be many reasons to call off the engagement... you just have to speak to the girl...)