Reading the first letter, my reaction is that it may very well be an OVER-REATION. Yes, you read that right.
Why was there a goy in shul? The answer is obvious: he was a guest. He was there for the ensuing wedding. Is it possible that these immodestly dressed girls were shiksas or maybe even non-religious Jewish friends/relatives also there for the wedding? Is it possible that, rather than reflecting declining standards of tznius in our communities, it’s an unavoidable consequence of what happens when Jewish families are “mixed” as to their religiousness.
Instead of being upset, maybe we should be all the more proud that the chassan and kallah chose the path they chose, rather than be influenced by these friends and relatives who live like the goyim