This does not weaken Juan’s argument. First, if there are more amateurs and they still rarely challenge professionals, that just strengthens the appearance of inequality. Second, this doesn’t address the question of how much resources actually make a difference.

This does not weaken Juan’s argument. Juan only talks about events where amateurs and professionals compete—we don’t know if that includes events where amateurs win. Also, even if amateurs sometimes beat professionals, that doesn’t rebut an argument about general trends.

This does not weaken Juan’s argument. The argument is specifically about the modern Olympic games (in other words, its domain is limited). This means that observations about the ancient Olympics are irrelevant.

This weakens Juan’s argument. If there’s no correlation between athletes’ access to resources and their performance, then Juan’s assumption that amateurs’ lack of resources hinders their performance is undermined. That removes support from the conclusion (i.e. weakens).

This does not weaken Juan’s argument. It just affirms Juan’s claim that amateurs have less access to resources than professionals, and doesn’t challenge the assumption that resources make a difference to athletic performance.