2. Assuming that Jyggylag and Sheogorath are the same entity (which is alluded to) how can you mantle one without mantling the other? And since there is no indication that you have mantled Jyggylag AND Sheogorath it leaves the door open for you having never mantled Sheogorath, merely his name/title.
....not to sound rude or anything, but did you even play the game? I think this point is explained pretty clearly during the Main Quest of SI.
Anyway, I don't see why this discussion is still going on in the first place. I think it is fair to assume that the Main Quest is completed by the Eternal Champion/Nerevarine/CoC/main hero of any TES game, while the optional side quests (guild quests, misc. quests, etc) are completed by SOMEONE, but not necessarily the main hero you play as. For example, while I supposed it is
possible for the Nerevarine to be the head of the Fighters Guild, Mages Guild, Thieves Guild, Morag Tong, one or all of the Great Houses, the Imperial Legion, the Imperial Cult, the Tribunal Temple, so on and so forth....I find it
highly unlikely that this is the case. Let's not forget why those quests are there in the first place; to give the PC (you) variety and choice in your gaming experience. If the only quest we could do in the games was the MQ, then it would certainly be easier to determine who did what, but it would also be a very boring game, as well.
Bottom line, I say just make up your own choice as to who-does-what where the answer isn't clearly stated. I mean, is it really that important who the head of the Vvardenfell Mages Guild is, anyway?