How can I get single keys in multi-item config objects using debug:config?
No, Drupal Console doesn't support selecting a key from a configuration object. This is clear from drupal help debug:config which even mention grepping.

Using grep isn't beautiful, but it works:

drupal debug:config system.date | grep -A1 timezone


does return the timezone with the first subkey. (Changing the value after A, you get more lines/the user subkey.) 

I would of course prefer to get the configuration object as proper JSON and then pipe it to jq to select a key robustly, but Drupal Console doesn't support an output format option yet - see Github issue.