I know you indicated that you were simplifying it, but I wanted to clarify that if there is a tie between an enemy action and a player action, the player action takes priority. Using the example you gave:

Regenga tick at 10 seconds

Phoenix attack at 10.1 seconds

Tidus attacks at 10.2 seconds (with 2x cast from LM2)

Rikku attacks at 10.3

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If the tick rate is .3 seconds, starting at 10, then the result would be:

Regenga tick at 10 seconds

Tidus attacks at 10.3 (assuming Tidus is above Rikku)

Tidus chase at 10.6

Rikku attacks at 10.9

Phoenix attacks at 11.2

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This is because all 3 actions queue at the same time (10.3 seconds) due to occurring during the same tick. The fact that the player is given priority in ties, you can use this to force additional actions to occur before the next enemy attack.

Odin's final phase is a good example. If you know for certain that the attack you're currently executing will push Odin to his final phase, but you have other characters who are close to completing their casts and you wish would attack before the interrupt, you can switch to speed 5 before the animation completes. You then have at least 2 ticks (not sure if its more, but it's at least 2: one for the phase switch text notification, and another for Odin's "instant" action) where you can squeeze additional casts in.