You know what, the Mistress of that household is basically a perfect wingman. I doubt that without her being as impulsive, playful and overall cheeky as she was, (filing a similar role to Ayasaki Remi in the Horimiya manga) Riho would never have confessed to Shun and just continued to despise him. I’m also 90% sure the mistress just wanted to either be cucked, or just wanted a suitable partner for Riho since she knew that without pulling this kind of stunt, she would’ve been lonely for the rest of her life