Minglan would have made the most out of her situation but she would have silently suffered a lot. She was willing to take in concubines when she was with GTY, she only spoke up for herself later with GTY because he understood her and he made her feel comfortable enough to do so. Yuanruo was much more submissive to his parents, so even if his mother forced in a concubine, I'm sure he wouldn't have refused and neither would Minglan. He wouldn't protect her like GTY did, which would cause a lot of heartache. Also because Yuanruo is such a bleeding heart, if the other concubines treated him well, I think he would care for them genuinely. Minglan's life would have been like a lot of women back then, sharing her husband with others and not being able to say anything about it. She would still have happiness, as Yuanruo was loving and kind, but she would continue to live being restrained by her mother in law and having to play games in order to keep the household in order.
Gu Tingye was 100% the right choice (even though it wasn't really a choice). GTY was the only man with the status and the personality that allowed Minglan to be herself and not have to compromise. I loved Yuanruo too, he was sweet and genuine, but in the times that they lived, Minglan needed someone strong and who understood her.