I'm not sure there's just this one way to read the bit about challenge vs. assassination.
Picard: "If I find Toral's challenge valid, the two of you will very quickly gain control of the Council and Gowron will be found dead shortly thereafter."
It seems to me Picard could be saying that the sequence of events is the following: Picard rules in favor of challenge -> a duel takes place and Toral of course is slain -> even the most "honorable" backers of Duras, seeing the direct path to throne formally blocked, flock under the wings of the Sisters for more covert action -> the Sisters now have enough backing to arrange for the assassination of Gowron.
If Picard does not allow the duel to take place, the Sisters have shot themselves in their feet, because the pro-Duras factions will now wait for Toral to reach the age and fighting competence to take the throne...
By the way if Gowron would had said here and now Toral would it had been owrite or do they have to do that in a sacred chamber at x pm?
Worf certainly reassigned the Chancellor's title without a formal appointment.
Perhaps there would be incentive to arrange for a formal spectacle in the Toral vs. Gowron case, as both sides would want good publicity more than victory. The Duras party would know they couldn't win the duel, so they'd want to milk on the public embarrassment of a grown-up warrior slaying a little boy.
Gowron would want to brush it all under the carpet as quickly as possible, of course. But he might also realize that if he did so, it might look dubious; better kill Toral in public, so that even the most reactionary forces in faraway corners of the Empire would feel that old-style justice has been served.