Add 2 form elements to Form via AJAX
Add the additional form elements in the build function. The build function is rerun on every ajax request. If you don't have other things to do as modifying the form structure, then the ajax callback can be empty and you only need to return the new ajax container.

So you need to change this:

Put the new form elements inside the ajax container (in the first function):

$form['options']['a-add'][...]


And then return the whole container in the ajax callback (second function):

return $form['options'];


You always have to return the outside ajax div, otherwise the ajax callback will not work a second time.