I find myself reading through these older posts and I come to the conclusion, surprisingly, that many of the more “conservative” opinions here are right (as regards tzniusdik dress and activity and alcohol consumption at kiddush).
Someone who is brought up with seychel and derech eretz will be taught to dress modestly and appropriately in shul, regardless of denomination. It all starts and ends in the home, so a parent will and should model this kind of dress and behaviour. Now, I don’t think either of the two letters in the OP were complaining about sleeves to the elbow and skirts to the knee. It seems a lot more than that was displayed. It seems that much more immodest dress and behaviour was described. There’s no place for this in anyone’s shul, or for that matter any place of worship.
I’ve also seen the growth of the drinking phenomenon, though I haven’t personally encountered much of it with the youth. And the immodesty of drinking can sometimes be of a gaaivadik variety – oh, I brought a $200 bottle of single malt…Oh, I brought a $300 bottle of vodka…etc. That is as untzniusdik as a low neckline on a dress, or worse, because it incorporates pride, greed and gluttony along with immodesty.
Our shuls are places of holiness, learning, and Jewish unity. If that is recognized by secheldik people, they won’t come dressed immodestly, and they won’t come to drink themselves silly.
I’d make a couple of exceptions and comments, though. First, often enough there are those who do not have the upbringing to know what is appropriate, and are wearing what to them might have seemed the most appropriate. If these are one-offs, there for a simcha, it is the responsibility of the baal simcha to try to educate them on expectations in a place of sanctity.
If the individual is in shul for the first or a rare time, it can be a teaching and learning opportunity even if they aren’t dressed appropriately. As hard as it might be, such an individual should be treated with respect and kindness. That way they may come back, and they would likely emulate the dress of those who go frequently.
Finally, where I vehemently do not agree is with the focus on women and their tzniyus responsibility, while indicating that the only male tzniyus responsibility is to keep his women caged at home. Each of us, man and woman has a responsibility to live a Torah life in the world, meeting the challenges of tzniyus and all other challenges with the education, mussar, and guidance of our parents, teachers, and those who we take as our leaders. We should all turn inwards to protect the vitality of our homes and our families and their holiness, at the appropriate times, in the appropriate ways. And in that, as in so many other ways, our mothers, sisters, wives and daughters can show us the derech.