Regulus rebelled against Voldemort, mainly as Voldemort was planning on harming someone Regulus loved (Kreatcher). Also most likely as Regulus was beginning to see his brutality. Narcissa went against Voldemort to save her son, once again not necessarily going against pure-blood ideology itself.

Because Voldemort isn’t the epitome of pure blood supremacy. That existed before him, and unfortunately it still existed after him. All the pure-blood ideology supporters also didn’t support Voldemort. None of the Black family members took the mark besides Bellatrix and Regulus. Many saw him as too brutal, and he was also a half-blood. I love reading fics where many ancient pure-bloods are actually against him, and don’t think Voldy is the right person to represent them.

Voldemort’s views are also very extreme, and cult-like. He really doesn’t represent what regular pure-bloods think, which obviously doesn’t make pure-bloods views any better since bigotry is bigotry. All I’m saying is that Voldemort doesn’t fully equal pure-blood supremacy.

We don’t know much about Regulus, other than he admired Voldemort, he was proud to join the death eaters. He only didn’t know what he was joining, because that’s how cults work. And I feel like that part of his characterisation is often forgotten which is funny since it’s the only thing we canonically know about him (although I have to admit I’ve never read a Regulus-centric fic). Narcissa cared about her family and nothing else. She had a son and a husband, was pulled into that madness, all she wanted was to get them out safely.

I don't think it was ever stated anywhere. We all just assume so. Like we canonically know almost nothing about Rabastan, but in my mind he's always had little brother energy. How does that make any sense, I don't know.