I found how I did it :
When you are in a boat with a mob behind you, if you leave the boat while pressing either A or D (to turn left or right) or both, the mob will take your place and continue controlling the paddles you used when leaving.
The mob can't actually move by using both paddles, but he can turn around.
And if you join back the boat, you keep all the momentum you had when leaving (only if a mob was controlling the paddles as I explained before) .
Well, the Elytra thing was only a "btw" thing because I thought that lots of people reported it. Doesn't seem like it. I'll create another ticket for it.
For the skeleton bug, idk how you knew that I played the map on another version than the 15w42a. In fact, I only used the 15w41b on the same map (before using the 42a) .
Screens showing the zombie controlling the paddle.