Did you set the "Instant" on the feat.2da to 1? That should make it instant without waiting for the end of the turn.
It may not. Combat in NWN2 works off "rounds". It is a turn-based game, after all, even if the turns run continuously. "Instant" on a spell only means it will cast at the start of the round. But each spell/attack still has to be queued up and executes one "attack" per round.
Now, characters can get multiple standard melee attacks per round. A level 20 fighter type is going to have four attacks per round. But that's still one attack "action" in the queue. And doesn't sound like what the OP wants.
When it comes to spells, they're still only going to execute one spell per round.
There are ways to circumvent a part of this restriction: with buffs, you can use a sequencer rod or rod of preparation or some other player-created item that allows multiple buffs to be instantly cast on the character at the start of a round. That's the only way I know to get multiple spells off in a single round, but they are defense only.
For creatures, you can hard-code their AI so they cast multiple buffs instantaneously at the start of combat. But again, doesn't sound like what the OP is after.
It may be possible to re-code and override some of the default spellcasting scripts so that certain spells could be cast as a unit, but that would probably be a considerable undertaking. You're effectively trying to bypass a core game mechanic. You're going to fight the game engine every step of the way.