DY: In response to your first point, you are working within a system in which your conclusion fuels your proof. Yes, being right is most important when it comes to following the Torah. But if you can’t prove that your way of following the Torah is correct (or, as is the case here, that someone else’s is wrong), how do you know that the other person is wrong? It would seem to me that baselessly denouncing other Jews’ practices as unacceptable is a violation of Lashon Hara, Sinas Chinam, Ahavas Reiacha, and probably others.
To the second point: Off the top of my head, peanut oil as Kitniyos, R’ Akiva Eiger on how to give a Get, the different Kinyanim by Mechiras Chametz, Davening Nusach Sefard, the Mesader Kiddushin not drinking the wine under the Chuppah. It happens all the time. That’s an integral part of the Halachic process- that the Poskim continue learning and can be Mechadesh against what the accepted Dinim are. They just have to be really sure of themselves for a major Chiddush.