Always optional You choose how many creatures to tap (including zero) Do multiple convoke abilities stack
If either the opponent or the permanent you control becomes an illegal target by the time Iroh's second ability resolves, the ability does nothing and you cannot choose to have the opponent gain control of the permanent
If a permanent card that would be returned to the battlefield face down by Yarus, Roar of the Old Gods's last ability can't be turned face up for some reason, it'll still be returned to the battlefield, but it will stay face down
Among main decks of Izzet Prowess builds specifically, 6 players are running four copies, 75 are running three copies, 32 are running two copies, 8 are running one copy, and 19 are running no copies of Vivi Ornitier
3 Tabletop Simulator Real-World TCG, Virtually Played