Well, you raise many worthwhile discussion points. Let me try to respond to a few.
As concerns the definition of modesty, I recommend not so much a dictionary as things like the catechism or theological treatises. I don’t know what you might find online, so I won’t attempt links, but if you are willing to buy a couple of books, I think that Alice Von Hildebrand has excellent insight. There was also a woman on a replay of EWTN’s The Journey Home last night who deals with this subject. Naturally, the present pope’s writings would reveal more insight, too. Particularly Theology of the Body and Love and Responsibility
The larger concern I have there, however (not just WRT modesty but lots of stuff) is that the dictionary isn’t really the obvious place to look for an understanding of things that can be best illuminated by the perspective of faith and morals which the Church provides.
Why hasn’t the Church categorically addressed this subject at greater length? Well, I suppose that question could be asked about a lot of things. It would appear that the Church tends to widely respond primarily to questions only when they become sufficiently widespread in their problematicness. Afterall, it’s not ultimately required of the Church to specifically respond to every single possible difficulty which exists in society in a more formalized way.
Further, we have to be able to use our reason to deduce some of these things in lived experience of life. That’s just an expected responsibility, I think. Being a responsible adult Catholic means more than simply obeying a set of formal guidelines from on high. We must be able to think things through and apply the principles with our reasonable faculties based upon what we already know.
As to the “physical” matters, I think that in a certain sense the Church has always focused upon these things, but different periods of time have merely found different emphases. The fact that, in recent decades, there may have been a heavy focus on one aspect, perhaps to the detriment of another, is just an unfortunate reality of life and perhaps something of a natural cycle. Of course, that’s not to say that the Church can’t always do better.
What someone does in their bodily expression is a reflection of an interior disposition. While it wouldn’t necessarily be inherantly evil to wear clothing of the opposite gender or use a different name, one must ask why the person is doing that. Milton Berle put on a dress for comedic effect, for instance. Was it sinful? No. But if one is doing something (like using another name or wearing clothing appropriate to the opposite sex) for reasons which go against their very nature as a person of a particular sex, then they are violating their own dignity. This is, ultimately, what usually occurs with those who fall under the various categories of transgenderism in it’s many forms.
Should it matter whether they do it in private or public? Well, I suppose that there is a greater potential scandal and disruption to society if it is public. But Jesus reminded us that we can sin just with our very thoughts that are unknown to anyone else. So, even if something like crossdressing were done entirely behind closed doors (which, if you think about it in the larger scheme of things, it really isn’t entirely; seeing as how they have to get the clothes from somewhere and so are causing others to participate at least indirectly in their actions), sure it’s still a problem.
