1) For this you would set the border to none, then add a heading with an <hr/> tag under it.
2) the custom buttons should work. If not, you may have to change the triggers for the Server Validation and and Universal Email behaviors to use the Button Subbmit trigger. Go to the Server behaviors panel and doule click the server validation server bahavior to reset the triggers.
3) On the server behaviors panel, double click the universal Email server behavior and set the Go To Page to go to the thank you page.
4) leave the from field as a valid email address from your domain. but click the from button to go to the advanced from screen, here click the lightning bolt next to reply to and select the email address form element.