I'm surprised to be saying this but I kinda agree. According to my notes, DM bought that farm for $835K cash in May 2011. That was around the time he had those non-traditional (trying to be polite) engagement photos done, and more than a year before LB went missing.
I'm not aware of anything that suggests the fiancee was involved in any of DM's nefarious activities. In 2011, there was a house in Oakville where they reportedly had lived together for at least some period of time (per my notes, the house was in fiancee's name and post-breakup, DM was involved in its sale in February 2012).
Just to add some perspective to the timeline, at the end of May 2011 (not long after buying the farm and the engagement photo-shoot), DM and his two buddies drove down to Mexico for the Baja 500 race (~3 week trip). By August 1, 2011, DM and CN had already gotten together.
I would be surprised if the ex-fiancee was called as a witness, but I do wonder if she was interviewed by police at all just for some background on DM. I think she was living in one of his condos at the time of the murder(s), so likely was still on good terms with him.