Do philosophers ever create philosophical fictions akin to legal fictions in order to refute an argument? If so, how pervasive is this practice?
Yes-- all the time. Take for example Robert Nozick's idea of the pleasure machine. Nozick asks - if there were a machine that you could just to be hooked up to that would guarantee that you have an entire life of the highest pleasure - but which would take you out of the hurly burly of everyday existence -- would you want to be hooked up? The vast majority of people say NO - which Nozick takes to prove that most of us believe there is more to happiness than pleasure. But yes, philosophers frequently use fictions/ thought experiments to either make or refute an argument.