When 'cron_safe_threshold' is set to 0 in settings.php, should the dropdown value on admin/config/system/cron be "Never"? or is it irrelevant?
This setting is for Drupal 7. In Drupal 8, the equivalent is in the config management system as automated_cron.settings.interval. To override it, like in Drupal 7, you can add:

$config['automated_cron.settings']['interval'] = 0;


to your settings.php.

To address the other question, the relevance of automated_cron.settings.interval &amp; cron_safe_threshold is the minimum amount of time (in seconds) since the last cron run that Drupal is allowed to run again. This value is checked (D7 &amp; D8) for a 0 threshold which it will interpret as FALSE and thus never run cron.