Yes taking turns picking, not decrease the number and then pick, and RANDOM draft is what you describe first, the normal draft method is taking turns picking.
And how can you say anything based on the number of demigods? Do you have the number? Hell, do you have even an official estimate?
In random draft it picks RANDOM HEROES which you pick from.
And DotA has multible heroes to fill multiple roles. There are numberous nukers, numberous healers, numberous support classes, numberous tanks.
Will Demigod for sure have two nukers, so each team can have one? Torchbearer is one. We know of no other nuker.
We only know of one healer, Sedna, too.
So one team could get both the nuker and healer, and the other team doesn't?
Unique would be HARDER to balance. NOT easier. Keep it out of ranked matches is all I'm saying.