1. The mobiles do have a return button. It's usually on the right hand side of the on-screen keyboard, can also be labeled "submit" or "go" -- and pressing it submits the input same as hitting Return on a keyboard. If it doesn't for you, I would question what device you are testing it on, your version of Unity and your version of NGUI. Even if you didn't, you can always add a "done" button to the screen itself. Most forms do. Username/pass, and the "Login" button for example. Clicking this button will just grab the values of username/pass input fields and proceed with the login action.
2. Change your input label's wrap settings.
3. When you get OnSubmit on the email, automatically select the password field. When you submit on the password field, automatically login.