You can only be culpable (or rewarded) for your own acts of will.
So, I somehow need to replace the accidental bump with something more sinister…
Actually, you have done this with the fourth man and having the bystander respond to the act of purchasing the magazine, instead of the bump. Say a man buys the magazine, and nothings happens to anyone else. Is the man still guilty of two sins, the one of buying the magazine, and the other of running the unnecessary risk of scandal? It seems pretty obvious that buying a magazine of this sort runs a foreseeable risk of someone seeing you. The cashier, at least. So the man buys the magazine, and either nothing happens, or a spectator turns away from sin, or a spectator turns to sin. In each of the three cases, is there any difference in the gravity of what the man has done?
You may argue that I did not will that my example would tempt another to sin, but I did will to commit the sin and in willing this, I set the example that later tempted another.
I think in a way, when you do a sin, you are also willing the consequences to others that come along with it. I know, not direct willing. But you are responsible for the consequences that you foresee or should foresee as likely. Since you are responsible for them, if you decide to say, “So what, I don’t care,” then you willed to say that, and it is wrong to not care about others. It almost seems like a separate decision from the one to do the sin by itself. Hmmm, maybe this is arising out of a positive duty toward your fellow man?
The contrast that underlies my previous paragraph is, what about the extremely well-trained Catholic who decides he is going to use/buy the men’s magazine. He does know that he will be causing negative effects in his own psyche, like creating an attachment to doing this particular sin. But no one ever accuses him of willing that damage to his psyche (well, except me when younger, so maybe my history is why I can’t figure it out). Yet in the paragraph above, I really want to accuse the man of some level of willing consequent harm to his neighbor. I want to pin something on him for the scandal.