This should do the trick…
<?php
$number_of_parents = comment_counter($post->ID);
$number_of_children = $post->comment_count - $number_of_parents;
?>
<?php if ( have_comments() ): ?>
<h3 class="comments-title">Discussion:
<?php if ( $number_of_children > 0 ): ?>
<?php echo implode( ' ', [
printf( _n( '%d question', '%d questions', $number_of_parents ), $number_of_parents ),
__( 'and' ),
printf( _n( '%d answer', '%d answers', $number_of_children ), $number_of_parents )
] ); ?>
<?php else: ?>
Questions: <?php echo $number_of_parents; ?>
<?php endif ?>
</h3>
<?php endif ?>