Need assistance with pre-headers.
First, there a couple things to understand about how a pre-header works:
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.
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.