The thing with Grandpa is the perfect example of the cultural differences between the American and the Japanese readers of the series. For the American readers, the fact Mei was still in an engagement comes off as poor writing when in reality, that was something that was always present according to the Japanese customs, and yeah, from his perspective, he was only doing what was the best for his granddaughter by ensuring she was together with someone who would support her, unlike Amemiya.
Yes, it was wrong for Mei to not say anything to Yuzu but that is also part of her characterization. Mei is striving first and foremost to uphold the family traditions as a means to make up for what she perceived was a failure on Shou's part so she didn't know what to do but to run from Yuzu. Ironically, this meant she also made the exact same mistake as Shou did.