AAQ, don’t worry, I’m not offended at all. Not sure what you mean by ארוסין being a social practice with a halachic basis – I mean, I suppose קידושין is a social practice with a halachic basis, but then so is חסד and שבת, which were your examples.
You say that you believe דעת תורה is a halachic imperative, so it needs to have halachic parameters. I have never heard of דעת תורה as a halachic imperative. I just think the concept means that it is sensible to follow advice from a גדול, because he knows better than you do, and can see things you cannot. The concept of taking advice from Gedolim about matters unrelated to halacha already existed in the time of Chazal (as can be seen from the Medrash I brought); it just wasn’t called דעת תורה back then.
In the Gemoro in Kiddushin, Ze’iri did not approach R’ Yochanan for advice. R’ Yochanan started a conversation trying to persuade him to marry his daughter. Obviously Ze’iri rejected R’ Yochanan’s advice, because he remembered the statement of R’ Elozor that R’ Yochanan had forgotten, so he knew his teacher was making a (halachic) mistake. And to answer your questions, if you come to your Rov and ask advice for a shidduch, I believe it’s perfectly ok halachically to reject it; however, it might not be a smart thing to do. And it certainly is ok not to ask if you know the answer yourself, or to reject a Rov’s advice if you know for sure he is wrong.