13 Jul 2018
Any cards not cast, including land cards, remain in exile.
13 Jul 2018
Casting an exiled card causes it to leave exile. You can't cast it multiple times.
13 Jul 2018
If an effect copies Apex of Power, the copy wasn't cast at all, so you won't add ten mana.
13 Jul 2018
Apex of Power doesn't change when you can cast the exiled cards. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty.