Scenario 1: Don't expect good feedback if you don't plan to give any. I've seen this happen plenty of times. It's not a crime to leave early or skip all other presentations, but it certainly doesn't make you a generous scholar.
Scenario 2: The Peacock. This one really grosses me out.
Student B: looks agitated throughout the student’s reading, and can barely sit through the next two papers. His hand shoots up during the Q&A session [or, in economics, well before]. 'Yes, this isn’t a question, but rather a comment, for the first reader–I’m sorry, I didn’t get your name. [doesn't bother to check program or wait for the student to tell him her name] But in listening to your essay, I wondered if you had considered... read... knew that X wrote an article on the subject in Very Important Journal……realized that your point has already been made by these 5 other Important People'...We should carry peacock feathers to wave around when this happens, so people can have a visual cue that they’re engaging in the vain behaviors of the peacock, grandstanding away, parading their knowledge for the world to see.
So, I agree that peacock feather grandstanding should be called out and reduced. It happens a lot (especially in economics?) and its gross. I don't go to great lengths to not embarrass people, however. If someone hasn't read a crucial paper in their field that is extremely relevant to this paper, I would let them know without hesitation. I'm not gonna avoid corrective comments completely, but I'll employ the normal-person manners that my parents taught me. But likewise, I don't expect others to go particularly soft on me.
But concern for sensitivity aside, during a presentation, we should be contributing constructive comments, not insulting one another or promoting our own agenda. And having presented to sociologists, historians and political scientists, I find that economists tend to be harsher than folks in other fields. I think this is partly because we interrupt each other. While I actually prefer interruptions as opposed to holding questions all the way until the end, we really do need to keep our grandstanding in check. Sit on your plumage and have some manners.