With Cooper's help (in Pt. 1), Ben's group retrieves Darkstar from the Null Void. Since they need all the help they can get, it's curious why they didn't think to contact Grandpa Max about getting some help from him, too (and thereby, Manny, Helen and Pierce), or a least check out how he's/they're doing.
Ben hardly fulfilled Max's instruction to actively seek Plumber offspring. In 'All That Glitters," Kevin's badge shows eight blips, revealing the locations of other Plumber badges and each potentially leading to other Plumber offspring. Only Alan Albright and Darkstar were tracked down as six leads were ignored. In "Ready to Rumble" (back in the original Ben 10 series), Ben met Gatorboy and Porcupine, who, being half-human, were very likely the offspring of Plumbers, too. They've never been mentioned since.