Custom field name not appearing in (strong) testimonial

Sorry, I should have tested it first.

Try this:
(your code was missing a closing </div> so double-check that)

<div class="strong-container <?php echo $container_class_list; ?>">
	<div class="strong-content <?php echo $content_class_list; ?>">
		<?php while ( $query->have_posts() ) : $query->the_post(); ?>
		<div class="<?php echo $post_class_list; ?> post-<?php echo the_ID(); ?>">
			<div class="inner">
				<h3 class="heading" style="font-style: strong; italic;"><?php the_title(); ?></h3>
				<?php /* The Content Block */ ?>
				<div class="content">
					<?php /* Thumbnail */ ?>
					<?php /* Removed in favor of if/then/else below */ ?>
					<?php /* See original testimonials.php in plugins */ ?>

					<?php if ( has_post_thumbnail() ) : ?>
					<div class="photo"><?php the_post_thumbnail(); ?></div>
					<?php else : ?>
					<?php echo get_avatar( wpmtst_get_field( 'email' ), 150, '' ); ?>
					<?php endif; ?>
					<?php /* Sets review exerpt to 200 characters */ ?>
					<?php wpmtst_field( 'truncated', array( 'char_limit' => 200 ) ); ?>
				</div>
				<div class="readmore"><a href="<?php echo get_permalink(); ?>">Read more</a></div>
				<?php /* The Client Block */ ?>
				<div class="client">
					<?php /* Custom Fields */ ?>
					<div class="name"><?php wpmtst_field( 'client_name' ); ?></div>
					<div class="company"><a href="<?php wpmtst_field( 'company_website' ); ?>" target="_blank"><?php wpmtst_field( 'company_name'); ?></a></div>
					<div class="city"><?php wpmtst_field( 'city_name' ); ?></div>
				</div>
				<div class="clear"></div>
			</div>
		</div>
		<?php endwhile; ?>
	</div>
</div>

Add this to your child stylesheet:

/*
The theme gives <h3> a weight of 500. Try 700 for traditional bold.
The plugin adds the quote symbol as a background image.
*/
.strong-container .testimonial .heading {
	font-style: italic;
	/* font-weight: 700; */
	background: none;
	padding-left: 0;
}

And now the shortcode only needs to be:

[strong per_page="5" nav="before,after"]

because the template handles the rest.

Edit: Removed get_the_ID() from has_post_thumbnail().

I don’t understand. Tell me in simple language what you want for the photo.

Me too. In testimonials.php, try

the_post_thumbnail( 'thumbnail' )

or

the_post_thumbnail( array( 150, 150 ) )

A big part of the learning curve is learning to think “Is there a WordPress function or option for that?”

When I said “Me too” earlier, I meant my operator error, not yours 🙂

It’s looking good. The full post uses the single.php template. You can copy that from the parent to your child theme and perform similar customizations as you did for the testimonials page. If you want your customizations to only apply to testimonials, not regular posts, name the file single-wpm-testimonial.php.