3 ballet questions :), please answer?

I have that problem too. Do your splits against a wall and have someone walk you into the wall. I know it sounds weird but trust me it's amazing. As for your posture I saw just keep correcting it and eventually it will naturally happen. That's what I had to do. Try breaking in your shoes better. If they are broken in then it's very easy to get all the way up. I just got my third pair and they are still stiff and if I didn't have a strong arch I wouldn't be able to get all the way up. For your arches go up on demi pointe everyday and push over in plie. Do this everyday along with normal ballet exercises and your arches should gradually get better.