I like your analysis. I think one of the under appreciated aspects of this mugging is the time limitation. 'Post in the next X minutes' is key to 'Choice C':

Allow[ing] an opportunity to avoid a large amount of (fake) disutility to slip away.

Specifically, it makes the person have to think about this with a limited time span and decide on what they perceive to be the action of lowest possible danger, with the [spectacularly gruesome details] of extremely low probability seeming to be more danger than wasting some time. It also has the effect of spreading the meme more quickly.

If we assume Y% of people who read the comment would spread it in each generation, the total # of times it is viewed is dictated by the gap between each 'generation' of people who spread it. By forcing the mugged to spread the meme within Z minutes, the meme becomes:

• 'Bumped' in a message board, granting greater visibility

• Spread with a smaller gap between generations, and so more effective at propagating

[Edited for proper formatting]