That statement isn't an argument, it just means "this hasn't been tried yet." If a dead god that refuses to stay dead can be mantled, so can a prince. In this case, Sheogorath used his window of opportunity to mold a mortal into a new Sheogorath, because mortals are, for the most part, free from predictability.
"This hasn't been tried yet" is an argument. Granted, not a particularly strong one, but it does rely on the absence of precedent.
Why do you believe that a Prince can, just because a dead god can? A dead god may very well be able to simply because he is dead and therefore does not currently have an avatar.
Do you mean antithesis or anathema?
Actually, either would work, but anathema is a bit closer. Mortals are "set apart" from the Daedra in many ways not the least of which being their antithesis relation of predictability versus unpredictability.
Could be that no one gave your shrine some cabbage, some yarn, and a soul gem on your summoning day or while it was raining. You still could pray to yourself after SI.
Yes, you could pray to yourself, but considering that you actually had followers at the shrine, and that there were many other shrines which we can assume also had followers, I find it hard to believe that they never once attempted to contact Sheogorath.
Of course, Tamriel isn't you. Tamriel has limitations on those who are from Oblivion. Even MD wasn't completely there at the end of OB, that was just an avatar.
Which would make you nothing but an avatar as well, yet if you run out of HP in Tamriel, you die... very unDaedric of you...
A cop out, but I will allow it.
Not a Daedric Prince.