The autocomplete Form API has the following problems:

1. Adding options $options = array('multiple' => false); will render a multi-select autocomplete-form not a single-select. Workaround: Do not add the array key 'multiple' at all, then autocomplete will stay a single choice field.
2. $mform->disabledIf does not work with autocomplete
3. Given $options = array('tags' => true); will render a single choice autocomplete form, but once a value is selected, it can not be removed anymore. It can only be replaced by another value. That should only happen when the field is set to "required". So even, if this field is not required, it is somehow difficult to leave it blank, once a value is selected. See MDL-63125
4. Saving blank autocomplete formfields, will result in multiple problem processing the form data. So there is a dirty hack to process an empty form. The items have to contain this value: qf_force_multiselect_submission, it has to be filtered after form submission. For an example have a look at this commit: [https://github.com/dasistwas/moodle-mod_datalynx/commit/d09a2cd2279e1d97aa6bc2c2453e4da39efa3f76 - see MDL-64151

Before start any work here, please read my comment below, if might be the case, please split the fix into small issues to limit the scope.