The only reason they brought back the original actor was because the Benzite makeup was meant for his head, so they brought him back to cut costs.
I think the writer did a good job maintaining the continuity. Mendon would not have been a real Starfleet ensign who graduated from the Academy. They make a point to show how ignorant of Starfleet protocols he is.
Fast forward to DS9. In Apocalypse Rising, Sisko overhears a Klingon warrior who says he killed Sisko's friend from the Academy, a Benzite captain. Not possible if Mendon was the first Benzite in Starfleet, whoops.