I know a small bit of the MBE we do deal with a lot of them not as much as Detroit and Cummins. Is the code still showing the #6? If so I would say yes it definitely could be. Honestly I know it’s pricey I would always recommend all of them but I know it’s not always fees able to do that and correcting the problem. It absolutely could be the unit pump check the connections try to follow the harness and make sure it’s not the wires a lot of times when persistent reoccurrence people seem to think the part has failed again but bad wire, harness could be the problems. Make sure they replace the injector line they are not to be reused. It could be multiple problems but I would stay on the track your on. I mean it could as severe as cams or internal motor issues or as simple as bad fuel and other issues throwing the code and I can’t rule out the injector themselves if it’s a reman injector they can go bad at immediate install or last forever. I would start with inspecting the wires and unit pump make sure everything looks good and dry. Maybe pinched oring during install so browse around. See for harness and wire issues a lot of dealers have test harness where they just instal and just she how they run with those. I can tell you them Mercedes is the worst motor. But I did see people on here had good runs but there is always going to be those select few that will run amazing.
I still haven't figured out how to make a Sprinter happy. 
