Form inputs being disabed during the Ajax request
That functionality is built-in so you'll need to monkey-patch Drupal.ajax.prototype.beforeSend (found in misc/ajax.js).

Copy the entire code, but remove/modify this line:

$(this.element).addClass('progress-disabled').attr('disabled', true);


So in your module or theme's JS file:

Drupal.ajax.prototype.beforeSend = function(xmlhttprequest, options) {
  // Place all code from the original function here, except the line above.
}


YMMV, you may unwittingly introduce unexpected behaviour by not waiting for each request to complete.