I have a question about who is responsible, appart from the Project Manager, for determining the stakeholders´roles. I´m not sure if it´s only the Project Sponsor or any other stakeholder who can help in this activity.
It depends what you mean by "roles". Responsible for Stakeholder management is, in my understanding, the Project Manager, but he or she should get input from as many places as possible, as descriped in PMBOK 22.214.171.124, 126.96.36.199 or 188.8.131.52 on Expert Judgement
Moderators: Yolanda Mabutas, Ahmed Amin, Michael DeCicco, Rahul Kakkar, Fernando Jr Sinlao Lim, Jeremy Raelin, Mary Kathrine Padua, Yasir Mehmood, Chris Preziotti, Lazard Toe, ITBMC, MPM, CIPM, PMP, ITIL, Christian Winter, Kevin Nason, Michael Sumaquial, PMP, Mark Wuenscher, PMP