flexibility

It will be better to have warm up exercises, before proceeding to stretching of your hamstring muscle to prevent muscle strain.

It is requires training and a lot, and it is depend on your age and on your Genes.. And if you will do it slowly and Listen to yourself, your body, you shouldn't get heart.. Any way you can start with your most slip, line it, and try to make it far from day to day. You should also flex you legs one by one. do like a side T stand that you are standing on one leg and the other leg is stretched on 90 degree for the side. You can also search for "adductors stretching exercises"

I think it requires training. I mean daily. You just have to keep on doing the spilt slowly and steadily. For example day one maybe not a total spilt but just widening of the 2 legs, try to expand to a point the pain or discomfort(but not totally painful of course) is bearable and stay there for a count of 30. After 30 is up, return back to normal position so your joints will be relaxed and do it again. On a daily basis just do 4 - 5 times. I bet by the end of the 3rd week or month you'll be able to do a leg split effortlessly.