Nope shinnok will boss in both story mode and arcade tower. There about four towers. In injustice you had classic battle and super man was boss but other battles,characters kept changing. I think the same will apply with the other 3 towers.

Source? Or are you talking out of your ass?

Her ass

I don't think it will work this way, but speaking strictly out of intuition?

I think there may be more than one boss for arcade, one of which being Shinnok.

No sources. Just a feeling. I am likely wrong.

One thing you must know,there klassic towers and living towers check the factions trailer.