I got the explanation for the third question.
Group announcements plugin description says:
"plugin removes the BuddyPress default “What’s New” announcement textbox (see screenshot, below) from a group main page, and makes it available instead on a new Announcements tab on the group’s sidebar. The functionality is identical – announcements that a member posts go directly into the group’s activity stream. The plugin addresses the confusion members face when posting to the forum, and accidentally posting an announcement, and clarifies how new discussion threads should be initiated."
Though, we would like to be able to share to a group from the profile page and to insert media at the group announcement level.