I would recommend that rather than Script Editor or editing the page directly that you actually add a Content Editor Webpart. Have it point to a .HTML file that you put in your SiteAssets directory. In the Content Link section of the Content Editor Webpart you can then include the HTML & JavaScript.
To overcome the Save it might actually be easier to just create the link to the list (new form) by creating a link on your site and passing a different source.
IE: if you have a button on the page that goes to http://yourfarm.sharepoint.com/sites/yoursite/Lists/YourList/NewForm.aspx?&Source=/sites/yourSite/Pages/Thanks.aspx
That way when they save it'll dump them off there. The cancel you could do a little bit of jQuery to change the function. I think the one item she was missing is you need to have that function but then there needs to be some code that overrides the default cancel.
<script>
$("input[value='Cancel']").attr("onClick", "goToByePage()");
</script>