Sweetness,
It sounds to me like you are handling this perfectly. By making it clear to your DD that it is NOT appropriate for her to make comments about others in public, you are teaching her not to be rude. BTW - I would not blast a 12 yo for making a comment, but if her parent didn't point out that she was being rude, I might simply because it would occur to me that she was a child and might not realize how she was coming across. Although I was raised that it is rude to point out when someone else is being rude, so I'm not sure I would say anything.
I also think that encouraging her to speak her mind to you privately is a good thing. This gives you a chance to reinforce your values to her. When she says to you (later, in private) that she thinks that girl's shorts were too short, etc. you can explain to her how you feel about it and help her understand that people are different.
As for the second topic - we have a harness for DD, but I've hardly used it. She likes the stroller so I haven't needed it, but every child is different. We are in a play group with a little boy who escapes the stroller and is a real bundle of energy and I know if he was my kid I would probably use the harness much more simply to keep him safe. memymomonica - I think the key thing to understand is that what works for you with your DD might not work for everyone simply because children are different.
I actually don't think I would have been offended by the CM remark about the "seeing eye child", because I would have assumed he/she was making a joke, not judging my parenting. Although tone of voice may have had a lot to do with why you were offended, it's so hard to get those nuances in writing.
I guess I see nothing wrong with using the harness so I'm not real sensitive to what others think. I know I'm doing what I think is best for DD so what someone else thinks isn't all that important to me.