You may activate Soul of Windgrace's last ability even if it's already tapped. It will still gain indestructible.

If Soul of Windgrace enters the battlefield attacking, its first ability triggers only once that combat.

You choose the land card as the triggered ability resolves. Once the ability has begun to resolve, no player may take actions until it finishes resolving. Notably, they can't respond to the land you choose by removing it from your graveyard.