I actually think the bike ride is going to be BMs undoing. Because you're right, he almost has to go with your theory (or one of her being kidnapped while on the bike ride) - because if LE can prove the bike ride was staged, there are really only two options. Either BM did it - or someone else staged it, but in an attempt to frame BM (which really makes no sense at all).
LE was skeptical of the bike ride almost immediately - there has to be a reason for that - a reason we'll learn soon. I think BM made a big mistake in the staging of the bike and it's going to be a big factor in the case.
They filed premeditated murder on BM - which means they must have evidence of activity prior to SM going missing that points to planning (disabling security cameras, making odd purchases, disabling GPS, possible witnesses to threats/violence, etc.) BM would have to somehow explain all of those away as just unlucky coincidences.
I'm going to assume that there will be cell phone data that will be difficult for BM to explain (his turned off at strange times, hers not matching his stated timeline, and her phone's last ping almost certainly being in the same location as BMs phone).
We have no idea what physical evidence (if any) was recovered from the house or construction site. We don't know what witnesses may testify to. Not to mention, Barry's "alibi" on Sunday is gawd-awful. Pulled together at the last minute, strange behavior on his part - none of it makes any sense.
I'm of the mindset that there will be a mountain of evidence against BM and it's not going to be difficult for a jury to find him guilty. MOO, JMO.