A participant is limited to four active roles and one paper presentation per Congress. So, yes. You can accept this invitation as a discussant or paper presenter. In this case, you would have two active roles in the Congress.
Yes. You can submit an individual paper presentation and participate in a workshop or roundtable. Participants are limited to one paper presentation per Congress. However, roundtable and workshop participants are not considered paper presenters because these sessions do not have formal paper presentations.
No. An individual can only present one paper per Congress. If your name appears as a co-author, this counts as your paper presentation.