If a views page display includes a page with a block display from the same view, the breadcrumb is overriden

I am not sure I understand the problem. Block isn't one of the allowed display types (see _custom_breadcrumbs_allowed_display in custom_breadcrumbs_common.inc) so a block display shouldn't be able to override an allowed display (like page, calendar, or image_gallery).

Perhaps this is a more complicated view than I am used to thinking about. Can you describe it further and explain why the logic in the current code is fail to work for your view?

Suffering from the same issue. I have a view page with nodes list and 2 blocks from the same view displaying summaries of similar nodes list. Theese blocks also call hook_views_pre_render and overwrite my custom breadcrumb... As they don't have nodes in result - tokens don't work....
Patch supplied above fixes this issue.

I can confirm this error with the latest dev version too. Bug still exist
To reproduce the bug:
-create a page view (for example: listing node titles) and create a block view (for example a slideshow or slider)
-put the block to a region and make sure that block is available and present at the page-view
-create a custom breadcrumb for the view or for that path (doesn't mather, neither will work)
Visiting the page, u'll see no change, what i mean the breadcrumb is overwritten.

After going thru and checking the code, changed the "_custom_breadcrumbs_allowed_display" to

function _custom_breadcrumbs_allowed_display($display) {
  $allowed_display_types = array('page', 'calendar', 'image_gallery');
  if (substr($display->current_display, 0, 4) == 'page') {
    if (!(isset($display->handler->view->is_attachment) && $display->handler->view->is_attachment)) {
      if (isset($display->display_options['path']) ) {
        if (module_exists('panels') && panels_get_current_page_display()) {
          return FALSE;
        }
        return TRUE;
      }
    }
  }
  return FALSE;
}

then the custom_breadcrumbs_views died at the 'selecting the path' part. I uninstalled the submodule, and created a 'path' custom breadcrumb. For me that quickly worked, however i personally don't think this would be the right way globally. I just give you the right direction to find the source of the problem. Now I create all custom breadcrumbs by path.
Anyway, thx for this great module, if i can help in any way don't hesitate to ask!