You do the round off, which is a cartwheel but you land on two feet. Once you mastered that, you learn how to rebound which is after you do the round off you jump up. After that, you do your round...
In the round-off, you'll have to in-curve and hollow your shoulders while you go into it. That way your back won't arch while you are doing the round-off. And before as you start your back handspring...
Yes, it actually is because when you do just a plain back hanspring without a round off you don't have as much momentum. That is what is easier about it, you have more momentum.
Yes. Some people only have their round of back-handspring and can't do a standing back-handspring. It doesn't really matter the order you do things in gymnastics.