But determination of the correct disutility conversion functions is itself a philosophical problem that cannot be waved away, and it's impossible to evaluate that claim until those conversion functions have at least been hinted at.

You seem to have gotten hung up on 3^^^3, which is really just a placeholder for "some finite number so large it boggles the mind". If you accept that all types of pain can be measured on a common disutility scale, then all you need is a non-zero conversion factor, and the repugnant conclusion follows (for some mind-bogglingly large number of specks). I think that if a line of argument that rescues your rebuttal exists, it involves lexicographic preferences.