I think even a single person *could* pull it off...but I still doubt this is what actually happened.
But to your point, yes, if a single could do it, then certainly a team could pull off this feat -- perhaps even exactly as you speculated.
That said, if the bike was found *directly below* the bridge, then that means someone had to dangle the bike over the rail, drop it straight down, and hope for it to drift to a spot directly beneath the bridge. Again...possible, but doubtful.
I'm leaning towards this being the work of someone panicked. And that's what leads me to lean in the area of the possibility that the person who dumped the bike was not the person who abducted MS.
Theory: they found the bike, took it, then when they learned of the significance of the bike, hurriedly disposed of it. The (reportedly) detached handlebars/grips could have been because the bike had been disassembled with intent to sell; or, alternately, that it was disassembled because it was being transported in a car, and removal of these parts helped the bike fit into a smaller transport compartment. (i.e. not in the bed of a truck)