I can’t help you from the parental side of things - but I can help you from your daughters side, as you sound EXACTLY like my parents.
I met my husband quite young. We ended up engaged at 18, married at 22. Too soon to tell if it’ll last, but we are within months of our 10 year wedding anniversary, so I don’t think it was an obviously bad decision I don’t for a second think marriage at 22 is a good thing - both my husband and I have changed a lot from when we were 22. Partly through good luck/chance, and partly through a lot of love and respect, we’ve ended up growing closer, instead of further apart, during our 20s when we were figuring out who we were - but the stats were against us.
My parents did not like my then boyfriend / fiance at all. It was “stupid little things” too.
Like not taking his cap off at the dinner table (he was 17-20, and his family were quite different, so the idea that there was a hat/table issue had never been exposed to him).
Like not wanting to hang around getting to know my parents (hello - he was 17-20, he wanted to be getting hot and heavy with me, not being measured and found wanting by my parents).
Like not having a sense of humour (he did, it was just a little different from theirs, and again, he didn’t feel comfortable displaying it in front of people who were measuring him up disapprovingly).
On one hand - yes, their observations were correct. And yet, they didn’t see the things I saw in him. Things I found way more important than whether he’d learned a certain etiquette point. Things that have made my life immeasurably happier. Since they didn’t see the things I saw, their concerns held little weight for me. I heard them, and dismissed them as they were irrelevant - the “stupid little things” didn’t matter in the big picture.
And even if they were correct, and the relationship was doomed, well I would’ve learned a good lesson about myself, wouldn’t I? It wouldn’t be the end of the world. And if they had’ve been more direct in their concerns, believe me I would’ve pulled away from them in an instant. Because I was trying to establish myself as an adult - and adults make their own decisions instead of following their parents wishes.
Becoming an adult and pulling away from your parents is a process, and invariably the parents and the adult child disagree on where on the continuum they are! Also, believe me that you do not know your daughter as well as you think you do. She has her own internal world, where her opinions and values have been developing, influenced but not dictated by yours. My parents were convinced my husband and I were ill-matched, mostly because they had their own pre-conceived notions on who I was and what my future would be - notions which I did not share - though I did “try them on for size” at one time or another.
So, in case you haven’t figured out my advice don’t say anything, and do your best to not let her know you have misgivings. Stupid little things aren’t important in the big picture. She will find her way - trust in the great job you’ve done raising her! Don’t give her reason to pull away from you. If you’d seen genuine red flags that could endanger her it would be a different story, but these are only “stupid little things” that bug you. Everyone bugs someone with stupid little things!
Also, my parents poor opinion of my then boyfriend (that they hid badly) had another effect. Instead of slowly exploring my own opinions of what was best for me, with parental support and input, the fact that they disapproved of my boyfriend when I knew he was a good match for me, made me pretty much decide they had no idea about what was best for me, and therefore I disregarded their opinions on a huge range of unrelated issues (to my detriment, I might add!), after all, if they were so wrong on this important issue, how could I trust their opinions on other issues? You don’t want to push your daughter away, so respect that she sees something in him, suck up your mild misgivings and give him a break