Okay, saw your updated question, so answering on Dushashan, Vidura and Keechak
Dushashan - if he wasn't around, Duryodhan would have had 98 other brothers to pick from to attempt to molest Draupadi, so nothing would have changed
Vidura - no difference in the story
Keechak - the Kauravas wouldn't have had any clue about where the Pandavas were, and there would have been no argument about whether they were successful in their disguise. Of course, it remains an open question whether Indraprastha would have been returned. Duryodhan could again have invited Yudhisthir to a third game of dice w/ the same conditions, ad infinitum. On what grounds would Yudi have declined?
Dusshashan-- He was definitely needed to disrobe, I mean yes there were 97 others (Vikarna wouldn't have done it for sure) but at any go someone was needed to disrobe, whatever be his name. That is not how we could discredit a character, in that case one might say Arjun is not needed, Nakul could have been as skillful as Arjun and he might have done everything what Arjun did. Aside I don't think that others would have actually even helped Duryodhan since the Sabha Parva clearly mentions that excluding the Chandal Chaukdi, everyone in the hall were sad at the state of affairs (which definitely includes Vikarna+97 others). The only option to discredit Dusshashan is to understand could Duryodhan do all the stuffs Dusshashan did by himself? He could have definitely brought Draupadi to the hall by force but disrobe!! I don't think Karna would have been able to order Duryodhan the way he did Dusshashan. Dushy was like a younger brother to him as well so he was able to ask him, but I don't think he could have done that to Dury. Whatever friendship etc. they might have had I don't think anyone could order the Yuvraj of a royal court. Maybe Karna could have himself tried to disrobe Draupadi. So yes in that case Dusshashan could be considered dispensable.
Vidur -- had he not been there, Pandavas could have ended in the Lakshagriha itself, so yes he was very much important
Keechak-- Already said not much to add