Ajax Calls Returning Zero in IE/Firefox

Have been working on this for 2 days and can't figure out what is causing this to fail in IE (v11) and Firefox. Works fine in Chrome.

Read a lot of questions on here that covered caching issues with multiple Ajax calls, particularly with IE, but I don't believe that's the issue. This code is actually one of two forms on the page. Both are submitted via jQuery, when a "Submit All" button is clicked. I disabled the first form, though, and the second still doesn't function properly.

Here's the JS/Ajax code for the faulty form submission:

$("body").on( "submit", ".update-guest-email", function(event) {
    event.preventDefault();
    var form = $(this);

    if( form.find('input[name="email"]').val() == "" ) {
        return;
    }
    if (!formChecker(form)){
        return;
    }

    var dataString = {
        action : 'update_guest_party',
        data : form.serializeArray()
    }

    $.ajax({
        cache: false,
        type: "post",
        url: ajaxurl, // This is the WP AJAX handler.
        data: dataString,
        beforeSend: function() {
            form.find('input').prop('disabled', true);
        },
        success: function(response){ //so, if data is retrieved, store it in response
            if (response.data >= 1){
                form.append('<div class="center confirmed"><span class="dashicons dashicons-yes"></span></div>');
                form.removeClass();
            } else {
                form.find('input').prop('disabled', false);
                $(".errors").html("That's weird; it didn't go through. You can try again or <a href='mailto:email@gmail.com'>send us an email</a>.")
                $('.errors').slideDown(250);
            }
        } // Close success response.
    }); // Close AJAX.
}); // End form-submit.

Again, that functions as expected in Chrome. Firebug shows POST data correctly:

action          update_guest_party
data[0][name]   email
data[0][value]  brundeezy@gmail.com
data[1][name]   table
data[1][value]  dw_wedding_mgr_guests
data[2][name]   id
data[2][value]  149

But then the response is just a '0' (zero). Per Chrome, the response should look like this:

{"success":true,"data":1}

Any help is greatly appreciated! I also have this on a live server, if it would be helpful to see it in production.