The reason all the objects change is because event 4 is top level instead of being a sub event of event 3; this means that all S_1 objects are being selected rather than just the one with the same variable as the S_0 that collided.
You also needed to swap over the animations when the direction changes.
Here's an updated capx:
https://www.dropbox.com/s/e2ajrvpne0mo4 ... .capx?dl=0
If S_0 and S_1 are always created together I would recommend putting them in a container - that way you won't need to use the variable check to identify the S_1: