[Coda]

The verbal pronunciation, at least in American English, is the same as "Columbuses," yes. There seems to be some disagreement over whether the version without the trailing s is pronounced that way or pronounced as just "Columbus."

There is no time where "Columbuses's" would be valid. If they were the ships that belonged to the collective members of the Columbus family, then that would be "Columbuses'" and that is consistent across dialects.