I am surprised that most of the posters have allowed the statement that covering “knees and elbows is halocha” go by unchallenged. Check with the gedolei horabbanim and find out if this is unanimously accepted halocha.
One of our problems is that we forget that different communities and sectors of observant Jews follow different poskim. There are seriously observant Jews who we misjudge because they follow a poseik that does not share our assumptions regarding halocha. We immediately judge them to be “not really frum”. I suggest that you verify this with the gedolim whom you know – I did.
I think that the aveira of misjudging a fellow Jew is far worse than the avairos that people find so upsetting.