1) You would use Conferences Attended when you were present solely for educational purposes and not presenting anything. Poster/Presentation can be used whether you attend the conference or not, so long as your name is on a poster/abstract as an author, though if you did not attend you should make it clear that someone else did the presentation.
2) Yes, so long as this happened at an off-campus event. Campus research symposiums are generally kept with the Research description.
Also note, if the data presented was the same for each, you would only list the most prestigious venue in the header (Publication or Poster/Presentation) and then list other presentations of the same data in that same space.
If each poster was distinctly different, then each may go in its own space, if you have the room.
3) No. They would still go under Posters/Presentations, but you'd give credit to the actual presenter, and make it clear that you didn't attend.
You might consider alternatively, grouping the personal presentations together (even if not similar), and nonattended presentations together (even if not similar), and making the fact of your presence, or not, apparent from the name you give the activity to cut down on the needed explanations.