[Gnomasz]

Overall, it's
[A's turn starts]A does stuff[A's turn ends]
[B's turn starts]B does stuff[B's turn ends]
[C's turn starts]C does stuff[C's turn ends]
[A's 2nd turn starts]A does stuff[A's turn ends]
… and so on. A character's turn ends before the other character's turn starts.

So, in the orc scenario, the mage gets to cast Armor before orcs attack him.

Sleep spell is another thing because it takes 3 seconds to cast, so it's (for example):
A's 5th turn: starts casting Sleep (Concentrate),
B's 5th turn: casts Fireball (Concentrate),
A6: continues casting Sleep (2nd Concentrate)
B6: throws Fireball, A dodges, rolls Will-3 (see Distraction and Injury, p. Basic 236 or Magic 7), passes and continues casting.
A7: finishes casting Sleep (3rd Concentrate), rolls for it and B rolls to resist.
B7: might already be asleep.

Stun is a simpler spell with 1 sec. casting time, this would be:
A's 5th turn: casts Stun, rolls for it and B rolls to resist.

Powers generally work the same: within the same turn order (it's really more a thing about basic turn sequence than magic or powers). TK is a bit special in that you need to Concentrate to get another maneuver with TK in the same turn. So your character Concentrates for your TK to immediately Attack and grab enemy's throat, but it's still subject to rules for Choke Hold (p. Basic 404), so on your next turn you can Concentrate on your TK, and it immediately, during your turn, performs an Attack to crush the throat and deal damage.