-will The Leader be graduating just from the group, or from Hello! Project?
According to the official statement that was released by Up-Front (english translation has been linked here -
http://forum.jphip.com/index.php?topic=37432.msg1274707#msg1274707 ), she will graduate from both Morning Musume and Hello! Project.
-any announcement when the next leader of both potential successors will be decided or unveiled?
The next leader of Morning Musume will be one of the current subleaders, Fukumura Mizkuki and Iikubo Haruna. In terms of an official announcement, I would guess that will be made either shortly before or even during Sayu's graduation ceremony/graduation concert.
Hell, for all we know, they may announce that since they were co-subleaders, both Fuku-chan and Harunan will get promoted to be co-leaders.