Form with multiple buttons, submits using incorrect button

Problem/Motivation

I have a slightly complicated form setup. The form has an multiple text boxes, an ajax-enabled select list, a table with multiple lines and multiple form elements, including a delete button, per line, one add button and, finally, a Submit button for the form.

So, in summary, the form has multiple ajax-enabled "delete line" buttons, one ajax-enabled "add line" button and one regular "Submit" button.
The delete line and add line buttons are working like a charm. But the problem is that the "Submit" button behaves erratically. If there are no validation errors, the form submits correctly. But, if I, for example, leave out a "required" field, I correctly get the form error message saying that that field is required. Now, if I submit the form again with this same error, the form just resets and reloads.
I checked the $form_state['clicked_button'] in my "_form()" callback and saw that the last "delete line" button was recorded as the pressed button.
To further confirm that this issue is popping up because of multiple "buttons" on the form, I changed the #type of the "delete" and "add line" elements to "radio" and this issue went away.

The following are the code snippets for the button elements inside my "_form()" callback:

//Set form cache to enable dynamic addition of bill line items to form via ajax calls.
    $form_state['cache'] = TRUE;
    
    if(isset ($form_state["clicked_button"]))
        watchdog('mymodule_add', var_export($form_state["clicked_button"],true));

/**This is from inside a loop **/
  $form['button_delete'] = array(
    '#type'    => 'button',
    '#value'   => t(" "),
    '#id'      => 'delete-line-'.$cleaned_key,
    '#name'    => 'delete-line-button-'.$cleaned_key,
    '#parents' => array('line', $key, 'button_delete'),
    '#ajax'    => array(
        'event'    => 'click',
        'callback' => 'purchase_item_delete',
        'wrapper'  => 'purchase-items',
        'method'   => 'replace',
        'effect'   => 'fade',
     ),
     '#submit' => array('purchase_item_delete'),
     '#executes_submit_callback' => FALSE,
     '#limit_validation_errors' => array(),
     '#attributes' => array(
                        'class'   => array('button_delete'),
                        'onclick' => 'jQuery("input[name=\'mymodule_ajax_cmd_array\']").val("remove_line|'.$key.'");',
                      ),
  );

/** Loop ends somewhere here**/
//

    //The button to add more line item rows.
    $lines['footer']['add_purchase_item'] = array(
      '#type' => 'button',
      '#id'   => 'save-and-add-line',
      '#name' => 'save-and-add-line',
      '#value' => t('Add Line'),
      '#title' => t('Add Line'),
      '#ajax' => array(
          'event'   => 'click',
          'callback'=> 'inventory_purchase_item_add_ajax',
          'wrapper' => 'purchase-items',
          'method'  => 'replace',
          'effect'  => 'fade',
      ),
      '#executes_submit_callback' => FALSE,
      '#limit_validation_errors' => array( array('line'), ),  /**Only validate the $form_state['values']['line'] elements.**/
      '#attributes' => array(
                        'onclick' => 'purchase_add_button()',
                      ),
    );

//Finally, the form's submit button.
$form['submit'] = array(
  '#type'   => 'submit',
  '#value'  => t('Save'),
  #'#id'     => 'form-submit-button',
  #'#name'   => 'form-submit-button',
  '#suffix' => ' | <span id="cancel_button"><a href="../inventory-purchases-list">Cancel</a></span>',
  '#weight' => 1000,
  '#attributes' => array(
      'onclick'=>"return purchase_save_button();",
  ),
);

Proposed resolution

As I mentioned, this issue went away once I changed my "delete" and "add" buttons to radio (or checkbox, and I suspect anything that supports #ajax will work, except for "submit", "button" and "image_button") (but I really don't want to!).

I also noticed that Drupal "forgets" to add any custom js library when there was a validation error in the form. I coped with this in my custom validation callback by re-attaching the js files after checking if there were any validation errors:

function mymodule_inventory_purchase_add_validate($form, &$form_state) {
    inventory_purchase_common_validations($form, $form_state); /*Here, I set form_error() for any incorrect user entry element*/
    if(form_get_errors()){
        $path = drupal_get_path('module', 'mymodule_inventories');
        drupal_add_js($path.'/res/mymodule_inventories.js');
    }
}

BUT, this only takes care of any custom validations. Basic validations like missing "required" field values are handled before my _validate() callback and hence do not re-attach the custom js files.
I only mentioned this because if Drupal determines callback buttons based on an external js (fapi's js?), then this could also be the reason for this bug.

I am also attaching a part of the form highlighting the buttons mentioned. Please let me know if any more code/clarification is required to solve this issue.