“Cases in point: chalav yisroel, black hats, sheitles, separate seating at events, bugs in lettuce”
Cholov Yisroel is a din. You want kulas? There are kulas – many of which had to be followed because ein gozrim; there were not options out of cholov stam. Rav Moshe himself mentions Toronto which has it’s own mehadrin milk production and says in such a case, unless it’s significantly more expensive, one should be buying cholov Yisroel. It is significantly more expensive, so many are somech on that kula (among others). To claim it is a minhag or chumra testifies that you have no idea what you’re talking about.
Seperate seating at events “became an issue” once big events were popular. In a chasuna in Europe, a little more than both families attended a wedding – not the 200-500 person weddings that we have today. So mixed seating tended not to be an issue HALACHICALLY, because there would not be fraternizing. Today, there is – hence there is a problem. Not minhag – you can argue chumra (I think it’s pashut that it’s not – but I’m nobody).
Bugs in lettuce. I don’t even know where you’re going with this. More knowledge which points to something being a problem means you must check it. Even if generations before did not, if you have something that is halachically muchzak b’tolayim – it MUST BE CHECKED. Not chumra, not minhag.
Sheitels and black hats… I don’t even know where you’re going with these.