One solution that appears to resolve this conflict:
1. Search-Replace all references to 'breadcrumb_trail', replace with 'breadcrumb_trail_plugin'. So now, calling breadcrumb_trail in a theme means using 'breadcrumb_trail_plugin', rather than 'breadcrumb_trail'
2. change the filename breadcrumb_trail.php to breadcrumb_trail_plugin.php (so that now, there is not the conflict, since Hybrid's library is using its breadcrumb_trail.php, while the Plugin uses its own breadcrumb_trail_plugin.php)
In any Theme (other than Hybrid, of course) file using breadcrumb_trail:
3. change all references to breadcrumb_trail to 'breadcrumb_trail_plugin', to refer to renamed function in renamed breadcrumb_trail_plugin.php
That seems to work so far. Not too inconvenient, and it allows me to bounce between Hybrid and other themes without locking up WP.
(Further thought: if the Plugin is updated, then the search-replace will likely have to be performed on the update so that it changed to 'breadcrumb_trail_plugin')