If Mystic Reversal takes control of Discipline targeting Irelia, does Irelia Fervent still trigger and Irelia Blade Dancer still trigger if new targets—not Irelia—are reselected, does the original Irelia player still get to use their legend ability if the spell is taken?

To address your question regarding the interaction between Mystic Reversal, Discipline, and Irelia, Fervent, we must look at the timing of "choose" triggers and the mechanics of control changes.

The Ruling

Yes, Irelia, Fervent's ability triggers, and it remains on the chain even if Mystic Reversal is played.

According to FAQ #636 and FAQ #6315, "choose" triggers occur during the finalization of a spell (when targets are locked in). Because this happens before the opponent has an opportunity to play a Reaction like Mystic Reversal, the trigger is already placed on the chain. Once a trigger is on the chain, it is independent of the spell that caused it. Even if the spell is later countered or stolen, the trigger remains on the chain and will resolve.

Sequence of Events

1. Player A plays Discipline: They choose Irelia, Fervent as the target.
2. Finalization: The spell is finalized. Irelia, Fervent's ability triggers because she was chosen. This trigger is placed on the chain.
3. Player B plays Mystic Reversal: They target Discipline in response to the spell (but after the trigger has been placed).
4. Resolution of Mystic Reversal: Player B gains control of Discipline. They may choose new targets for the spell.
5. Resolution of the Chain:
   • The chain resolves in LIFO (Last-In, First-Out) order.
   • Irelia, Fervent's trigger (placed in step 2) resolves, giving her +1 Might.
   • Discipline (now controlled by Player B) resolves with its new target.

Addressing Your Specific Points

• Does Irelia, Fervent still trigger? Yes. The trigger occurs at the moment of finalization. It is already on the chain before Mystic Reversal can be played.
• Does Irelia, Blade Dancer trigger? Yes. Per FAQ #6256 and FAQ #220, Irelia, Blade Dancer triggers when a unit is chosen by a spell. The same logic applies: if the condition was met during finalization, the trigger is already on the chain and will resolve regardless of who controls the spell at the time of resolution.
• Does the original player still get the benefit? Yes. Because the trigger was placed on the chain by the original player when the spell was finalized, they are the "accountable player" for that trigger (Rule 506.1). The trigger will resolve for them, even if the spell itself is stolen and resolves for the opponent.

Note: I could not fully verify this ruling against the retrieved rules.