Very long post, and you make some valid points.
However, I will tend to agree with Oomis on this point.
You are correct that most relationships in high school do not culminate in marriage. Most of the time, it is broken off because the couple was dating for fun and to be cool, not for marriage. However, that is expected when dating at such an age. Most of the kids do not realize what life is actually about.
However, what I believe Oomis is getting to is more the social aspect. Kids, when dating and interacting with their social opposites when they are younger, learn life experiences and how to deal with the opposite gender better than when they are starting at a later age. IMO, You can not gain this experience talking to your cousins and brothers/sisters. When was the last time you took a sibling or cousin on a date? You interact differently with family than you do with someone you might plan on marrying/dating.
Furthermore, I would like to point out that you might agree with Oomis. You said that the boys/girls are not mature enough when they are younger to get together at social events. What I got from your post is that after seminary/yeshivah the boys/girls WILL be mature enough to go to social events that are not chaperoned by a Rav or is not a shiur.
You mentioned that you dated boys so I assume you are/were in the parsha. My question to you is how many of the boys actually treated you nicely on a date. Most of the bais yackov girls I went out with complained to me that their previous dates do not show an ounce of social grace when it comes to dating. However, my MO friends all agree that the boys know how to interact and treat them with respect instead of like a piece of meat (or a an enigma).
Dating at a younger age allows boys and girls to treat each other with respect when they get older and we wont have this “shiudduch crisis” (which IMO there really isn’t one) or crazy standards from boys and girls about what they want in a spouse.