Okay - so I'm going to go down another train of thought, starting with simply "what if BM is telling the truth"? So in this exercise, let's assume BM is innocent and is truthful in his YT "interview". Of course all this is MOO, JMO.
If that's the case, then SM was most likely last seen by BM on Sunday (BM didn't say when he left for Denver, but implied it was on Sunday) - not certain of the time. Though, if 911 was called at 5:46 because the girls were worried about not getting a response from SM during the day, one could reasonably assume that BM must have left a number of hours before then. We don't know, but for sake of argument, let's say sometime in the a.m.
So then that means that SM more than likely took a bike ride during the am or afternoon on Sunday. (it seems highly unlikely that anyone other than BM would have motive to stage the bike). For reasons discussed at length in this forum, animal attack seems highly unlikely. Accident seems unlikely (could have she been disoriented and wandered away, or in the river and washed away? Again, this seems improbable).
That leaves abduction. If we look at an abduction scenario, I believe we can make a few assumptions:
If abducted, SM would have almost certainly been placed/forced into a vehicle. It's hard to imagine a scenario where she would have been abducted on foot or ATV.
SM would have resisted this - meaning she would have either been unconscious, threatened (would a woman threatened at gunpoint get into a vehicle? I don't know), or she willingly got in the vehicle.
Could have a perp run her off the road, forced her into the ravine, dragged her unconscious body back up the ravine into a vehicle and taken off? We'd still have to explain the "personal item" found elsewhere (SM wakes up and throws something out the window? Perp throws something out the window?) Could someone do this on their own, or would it take two perps to get SM up the ravine (at least without leaving obvious evidence/drag marks, etc.) From the video, the ravine where BM said the bike was looked to be rather steep.
As someone speculated earlier, could have SM crashed and been hurt and willingly got in a vehicle of either someone she knew or someone offering help? If so, did that person then quickly incapacitate SM (and the perp threw the personal item out the window) or did SM realize what was happening and do that? I personally think this may be the most likely abduction scenario (especially taken in light her reported personality traits of being such a friendly, caring person who may have been too trusting). What I have trouble reconciling with this scenario is LEs actions that don't seem to be pursuing this as a likely option (given the lack of public appeals to be on the lookout).
Of course all of this assumes that the personal item that was found was in a place where it could have been thrown from a vehicle. If that's not the case, then I'm not sure any of these scenarios work to fit both the bike and the personal item..... and in all scenarios, we're talking about a broad daylight abduction of a grown woman who would have certainly resisted. Any scenario that would have involved a scuffle, screaming, etc. would have carried significant risk of drawing attention - it's remote there, but not completely isolated and we have seen from other posted videos that the number of hikers, etc. in the general area were likely fairly high.