I think Gabe is wrong about interleaved replies, clearly, but the rest of his advice is spot on. I’ve long thought email clients should only ask for the recipient at the bottom of the screen, or when you go and hit send.
I used to draft emails like this, but fell out of practice. Consider that changed now I am back to drafting outside of email apps (you guys will never guess which app I am going to use). (First iOS email app to add an extension system for “draft reply in…” wins.)