HTML being stripped/replaced by redactor on initialising despite being saved correctly
Have you tried using the allowedTags attribute in your Redactor config?

$('#redactor').redactor({
    allowedTags: ['div','p', 'h1', 'h2', 'pre']
});


You can read about that here:

http://imperavi.com/redactor/docs/settings/clean/#setting-allowedTags