Ditch a Hike

Spoiler

To successfully bump B off the platform, push him to the direction of the left stairs, not the right—less chance of falling over the edge.

If you rotate the draggble one full block (90°), the force will be too hard and wil just send B over the edge. Try to push B with the draggable open to just maybe 5°, like a sliver of opening, then push. That'd be enough to give him just the right amount of push.

Spoiler

Getting to the win: The autopilot trick is a bit hard here because there is little leeway for B to maneuver, but I've had much success with tapping a little further down the stairs (not directly beneath him), then have him climb the stairs again, then tap again to do the autopilot. Because B had just climbed, he will have to turn back to go down, and you can use that split moment to pull the win.