That's not the right house. The gemstone of darkness is on a glowing shelf in one of the other houses in Old Ashton.
1. You can access locations via the worldmap where you walk around, not via the overview map on which you build and upgrade constructions.
The overview map has a turn system. You need to use the "end turn" button to advance the turns.
2. On the overview map? Build a lumberjack.
3. Check the paperwork on the desk in your bedroom.
4. Fight vanguard battles. You get equipment and items as rewards for defeating enemies, opening chests, and winning battles.
5. That probably means that you're missing a prior step for Mira's events.
In that case, he can be found in the bar in Aldlyn. But that's where his story ends when you decide to return him to male.
You need the "Monster Craft Recipe" to craft those things, which you get from Vivian after the events of capturing monsters allowed you to build a monsterhall in your castle.
That comment by Trey is about Mirel's event. They continue in her events, not in Trey's story.