No both function cuz Itachi had one that worked and Danzo had one that workedYou don't know that.....maybe he had an eye disfunction and one eye looked the other way so he can't look at his enemy....maybe
He gave it to itachiHow the hell did Itachi beat Shisui anyway??? Or did Shisui just give him his eyes and ask for him to kill him? I can't remember...
Its his own technique which means that he doesn't have draw backs that others have...If he had 1sts dna then yes he can spam the technique.