Sorry I did not see your post until today.
I also came to a similar conclusion as you after watching this video:
by Locomotivist.
I did not use any empties or constraints - only bones and Ik’s.
Edit: to get the pivot point of the link sliding I created a third IK system, which points to the center of the radius of the link’s curve that maintains and slides the pivot point in the groove of the link. You can change gear by rotating the lifting arm.
I used Charlie Dockstader’s Simulator to synchronize the piston.
Here is a video of my results:
Here is my file:
WalschaertsGear.blend (1.5 MB)
This has been a fascinating project that has had me entertained for over a week!
Best regards.