Thats rly different points here, similar but different, the shadow council "was horde" , those entity is not
when there is no horde they need to chose one among then, they didn't trick someone to appoint him, they support him, more easy to manipulate, but intends to not make illegitimate
still, like i said before, why an illegitimate warchief have the power to put a legitimte warchief? evertyhing is ilegitimate
If the illegitimate warchief have followers and powers of course mak'gora is needed to validate himOnce they found out Sylvanas was appointed by tentacle monsters, they'd still need to go for Mak'gora if they want to challenge her on this basis.
If the illegitimate warchiefs rend and vol'jin fight, by example, the winner would be the true warchief.
IF Baine challenged her, and she wins, it would kill 2 rabbits for most of you guys, Baine die and she become legitimate , trough the horde law.