Pre-header visible even when selected to hide

First, there a couple things to understand about how a pre-header works: 

• Email clients (Outlook, Gmail) are going to display a "pre-header" regardless of whether or not you enable it in the Email Settings of the Marketo editor. 
  • If you do not input a pre-header into the email settings, the email client will display the first string of text (usually a max of 80 characters) that it finds in the body of the email. 
  • If you do input a pre-header into the email settings, the email client will display that before the text in the body of the email. 
• There are actually two ways that you can insert a pre-header into your emails:
  • Using the pre-header option in email settings
  • Including the pre-header in the code of the email - this is accomplished simply by adding the desired text to the code of the email as the very first line of readable text. You can add this using an element in the editor, by editing the code of the email, or by coding a My Token into that spot in the template. Some people style the text so it is not visible to the recipient. 

With that understanding in place, back to your questions: 

When I clone an email and try to hide the pre-header, it shows up anyway.

This is because, in the absence of the pre-header set in email settings,  the email client is pulling in the first string of text it finds in the email. 

When I make the preheader visible and fill it with info, it still shows a part of the old pre-header even though it's been deleted in settings.

My suspicion here is that what you are seeing is the pre-header you set in email settings followed by the old pre-header. Likely this is happening because a) the old pre-header is coded into the email and b) your new pre-header is short (less than 80 characters). If you have a pre-header shorter than 80 characters the email client is going to fill the remaining space available with the first text found in the email body. One easy way to get around this is to add a line break (press enter on your keyboard) at the end of the pre-header in email settings. 

• When I go into the code to delete it there, I get the error below in a section unrelated to the pre-header.
  • <div class="mktEditable center mktEditable center" id="footer" style="" mktoname="footer">
    You cannot use two mkto definitions in a single class
• After I save, it reverts the entire body of the email back to the original cloned email and I can no longer edit!! What am I doing wrong? 

Both of these issues are caused because you are deleting something you shouldn't be in the code. You will need to provide the original code and show the part you deleted in order for us to accurately troubleshoot that.