Well, we don't know for sure if Mickey died, even if that is the most logical conclusion, given his silence.
And how can they find out the truth? Probably interrogate the girlfriend and him against each other.
For example, there have been cases where victims have been left alive at locations by the perp. to access in the future. One example I can think of immediately is victims being left chained or tied to trees in a remote area.
So, maybe LE got him before she succombed.
LE is probably crawling all over any possible areas she could possibly be, to try and find her alive.
It has been my experience that a missing person is considered by LE to be alive until there is evidence to the contrary. It would be very optimistic to believe that a prolific predator such as Brandon Lavergne would remove the bicycle & other items from the crime scene, attempt to destroy all other incriminating evidence, yet let a victim live that could testify against him.