If I play Ride the Wind to love my Diana onto the Abandoned Hall battlefield, will I get the battlefield's effect to give my Diana +1 might? Why?

Yes, in this scenario, you will be able to give Diana +1 Might from the Abandoned Hall effect.

Ruling

As clarified in RiftJudge FAQ #10497 (marked as current), the "When a player plays a spell" trigger on Abandoned Hall occurs only after the spell has completed its resolution, not when it is placed on the chain.

Sequence of Events

1. Play Spell: You play Ride the Wind, choosing Diana as the target. Ride the Wind is placed on the chain.
2. Opportunity: Both players have a chance to play [Reaction] speed cards.
3. Resolution: Ride the Wind resolves. Its effect executes: Diana is moved to the Abandoned Hall.
4. Trigger Condition Met: Once Ride the Wind finishes resolving, the trigger condition "When a player plays a spell" is fulfilled.
5. Trigger Placed: The Abandoned Hall triggered ability is placed on the chain.
6. Resolution: You resolve the Abandoned Hall trigger. Because Diana was moved to the Abandoned Hall during the resolution of Ride the Wind (which occurred before this trigger was placed on the chain), she is currently at the Abandoned Hall and is a legal target to receive the +1 Might.

Note on conflicting information: Previous FAQs (#9723 and #9818) suggested the trigger occurred as soon as the spell was finalized, which would have made the unit an illegal target. FAQ #10497 is the current, authoritative interpretation for this interaction and takes precedence over those older rulings.

Note: I could not fully verify this ruling against the retrieved rules regarding the effect of Ride the Wind readying the unit.