How do I alter the comments form in several different ways?

The argument is an array, so you can use multiple key-value pairs, like this:

comment_form(
    [
        'title_reply'          => 'Leave a comment',
        'comment_notes_before' => 'some text',
    ]
);

In your second example you are using curly quotes. These are not valid PHP. Make sure you have the debug enabled during development, so you can see these errors before you push them to the live site.