If I'm holding Abandoned Hall and I play Discipline but my opponent plays Defy, and then I play Defy, does my opponent still get +1 for playing Defy on that battlefield even if I defy it?

No. Your opponent does not get +1 Might from Abandoned Hall — their Defy was countered, so it is not considered "played" for Abandoned Hall's trigger.

Here's the sequence:

1. You play Discipline. Opponent plays Defy targeting it.
2. You play Defy targeting opponent's Defy.
3. Your Defy resolves first (LIFO), countering opponent's Defy.
4. Opponent's Defy is removed from the chain without resolving — it's treated as never having been "played."

According to RiftJudge FAQ #11280 (and confirmed by #10646, #10093, #4905), a spell that is countered fails to meet the "when you play a spell" trigger condition on Abandoned Hall. The ability never goes on the chain for your opponent.

Your Discipline will then resolve normally, and you may give a unit +1 Might from Abandoned Hall when it resolves.