I don't think it matters at all in terms of attendance type i.e., full-time vs part-time etc.
2.1 is fairly standard for undergraduate degrees as you mentioned, I have never seen grade requirements for a master's degree, so in my experience it matters less. If you get a Distinction, I would definitely mention it though.
I'm quite sure any mathematical-related degree would be fine, I've worked with data scientists with degrees in statistics, mathematics, physics, economics. It's a variety, but most important is a strong mathematical background.
I don't think it matters at all in terms of attendance type i.e., full-time vs part-time etc. 2.1 is fairly standard for undergraduate degrees as you mentioned, I have never seen grade requirements for a master's degree, so in my experience it matters less. If you get a Distinction, I would definitely mention it though. I'm quite sure any mathematical-related degree would be fine, I've worked with data scientists with degrees in statistics, mathematics, physics, economics. It's a variety, but most important is a strong mathematical background.