If you have the PHx plugin installed and enabled, you will not be able to use the placeholders in your content like that. PHx eats placeholders in the content.
You can work around this by having separate Jot calls with different parameters to only show the form, or only show the comments. Or, you can use the Jot call you have, and write snippets to pull out the placeholders from the $modx->placeholders array:
$comments = $modx->getPlaceholder('jot.html.comments');
return $comments;
If the user is logged in, it will automatically use his username. You will have to customize your own tpl for the comment formatting. Be warned that Jot tpls use Jot’s internal PHx (not the plugin) and the tpls take a bit of studying to figure out.