This is full of errors and pure speculations. Constantine did not 'preside' over the council of Nicea. He merely told the bishops in essence, "you guys go figure this out, just come up with some standard we can pull the ranks in behind and administrate this whole thing". He had no theological voice. It was purely to bring a Roman-style administration into things, which is what happened.
And as far as the 'common people', the only reason the proto-orthodox groups which became the Catholic church proper took the day in this, is because they had in fact large followings. It's not like the masses believed something totally different, and suddenly they were forced to believe something entirely different. Yes, of course they were those who did, as there are today, but the point was to create a standard. Not to impose some foreign beliefs into this. They took the impetus of the stream and used it to standardize. They didn't create it.