the misunderstanding is in this section:
<?php if ( is_search() ) : // Only display Excerpts for Search ?>
<div class="entry-summary">
<?php the_excerpt(); ?>
</div><!-- .entry-summary -->
<?php else : ?>
<div class="entry-content">
<?php the_excerpt( __( 'Continue reading <span class="meta-nav">→</span>', 'twentytwelve' ) ); ?>
<?php wp_link_pages( array( 'before' => '<div class="page-links">' . __( 'Pages:', 'twentytwelve' ), 'after' => '</div>' ) ); ?>
</div><!-- .entry-content -->
<?php endif; ?>
it basically checks if the search result page is show, then shows excerpts, otherwise shows excerpts – there is no check to show a full post in a single post page.
possibly change the section to:
<?php if ( is_search() ) : // Only display Excerpts for Search ?>
<div class="entry-summary">
<?php the_excerpt(); ?>
</div><!-- .entry-summary -->
<?php elseif( is_single() ) : ?>
<div class="entry-content">
<?php the_content( __( 'Continue reading <span class="meta-nav">→</span>', 'twentytwelve' ) ); ?>
<?php wp_link_pages( array( 'before' => '<div class="page-links">' . __( 'Pages:', 'twentytwelve' ), 'after' => '</div>' ) ); ?>
</div><!-- .entry-content -->
<?php else : ?>
<div class="entry-content">
<?php the_excerpt( __( 'Continue reading <span class="meta-nav">→</span>', 'twentytwelve' ) ); ?>
<?php wp_link_pages( array( 'before' => '<div class="page-links">' . __( 'Pages:', 'twentytwelve' ), 'after' => '</div>' ) ); ?>
</div><!-- .entry-content -->
<?php endif; ?>