If I play Downstage Dramatics with the repeat cost, can I then play Jhin, Meticulous Killer for its alternative cost of 1 blue power?

Yes, you can play Jhin, Meticulous Killer for his alternative cost of [C] in this scenario.

Here is the breakdown of why this works, based on the rules for playing spells and the specific interaction with Jhin:

1. Total Cost and Spending: When you play Downstage Dramatics, you pay its base energy cost (2) plus the additional [Repeat] cost (2). This results in a total of 4 energy spent to play that spell.
2. Jhin's Condition: Jhin, Meticulous Killer checks the energy spent to play a spell during the current turn. According to RiftJudge FAQ #9920 (which also clarifies this regarding countered spells), Jhin checks if the energy was spent and the spell became a "Finalized Chain Item." Because you spent the energy to play Downstage Dramatics—including the additional [Repeat] cost—you have satisfied Jhin’s requirement of spending 4 or more energy on a spell this turn.
3. Spell Resolution vs. Spending: It does not matter if the spell resolves or is later countered; the energy was spent to play the card, which is the only requirement Jhin checks.

Sequence:

• You declare you are playing Downstage Dramatics from your hand.
• You declare you are paying the [Repeat] cost as an additional cost.
• You pay the total cost (2 + 2 = 4 energy).
• Downstage Dramatics becomes a Finalized Chain Item on the chain.
• Jhin’s condition ("If you've spent [4] or more to play a spell this turn") is now satisfied for the remainder of the turn.
• You now have the permission to play Jhin, Meticulous Killer for his alternative cost of [C].