Here's a sketch of my favourite solution of how to drive a sternwheeler by a hidden electric motor (two versions of variant 6 from the original posting):
Less parts than the classic scotch yoke, and truly homokinetic, so the drive won't jam oder tense up when both engine sides are quartered by 90 degrees.


And even better, here are animations of both versions:





In addition, here's an animation of a different, but also homokinetic solution using a rocker instead:



enjoy, Moritz