1. I believe it was in the time of the netziv; there was a gezerah against having long payos, and the litvishe poskim held that it wasn’t yehereg velo yaavor, and started putting them behind their ears, but the divrei chaim held that it was. The netziv was moser nefesh for it.
2. I don’t think it was general anti semitism; there were often gezeros regarding how Jews dress; the shtreimel is part of that history.
3. What is the reason some people curl their Peyos? No idea
4. Why do some wear their Peyos over their head/under their Yarmulka, coming out on the other side of their face? That’s the minhag by gerrer chasidim. Rav wolfson’s reasoning could apply here, but best ask a gerrer chossid.
5. How long should the Peyos *ideally* be, not considering antisemitism or other external/non-halachic factors? The satmar rov said that it shouldn’t be longer than the chin
6. What is the reason some people have *no* Peyos hanging from their sides (i.e. not even hidden behind the ear)? It’s a minhag, not a din, and once there was a hault to the minhag in many places(and in places like Germany, where the reason was less than noble), many families didn’t restart the minhag.