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For the left side you could do this,

<?php
$args = array(
  'post_type'      => 'post',
  'posts_per_page' => 10,   
  'paged'          => get_query_var('paged') ? get_query_var('paged') : 1,
);
$loop = new WP_Query( $args );

// add custom function before loop, so you can apply excerpt filter only to this case
do_action('my_custom_pre_loop_action');

// is url parameter set, if so force it to integer
$current_post_id = ! empty( $_GET['postid'] ) ? absint($_GET['postid']) : 0;

while ( $loop->have_posts() ) : $loop->the_post();
  $post_class = 'et_blog_post';
  // is the current loop item the same as the post in view?
  if ( get_the_id() === $current_post_id ) {
    $post_class .= ' active';
  } else if ( 0 === $loop->current_post ) {
    $post_class .= ' active' // make first post active as a fallback
  }
?>
  <article id="post-<?php the_ID(); ?>" class="<?php echo $post_class ?>">
    <div class="entry-content">   
      <h3 class="entry-title">  
        <a href="/blog/?postid=<?php the_ID(); ?>#post">
          <?php the_title(); ?>
        </a>
      </h3>
      <div class="post-content">
        <div class="post-content-inner">
          <?php the_excerpt(); ?>
        </div>
        <p>
          <a href="/blog/?postid=<?php the_ID(); ?>#post" class="blog-read-more">READ MORE</a>
        </p>  
      </div>
    </div>
  </article>
<?php 
endwhile;
// reset global $post after custom WP_Query loop
wp_reset_postdata();

And instead of setting the excerpt length with substr(), you can use excerpt_length filter. In the example above I added custom action which adds the length filter so that it happens only in this context and not site wide.

// apply custom exerpt lenght only when your custom action is fired
add_action('my_custom_pre_loop_action', 'apply_my_custom_exerpt_length');
function apply_my_custom_exerpt_length() {
  // if you want to apply this globally on your site then just move the add_filter to your functions.php
  add_filter('excerpt_length', 'my_custom_exerpt_length');
}

function my_custom_exerpt_length($length) {
  return 100;
}

And for completeness sake, example for the right side,

<?php
$current_post_id = ! empty( $_GET['postid'] ) ? absint($_GET['postid']) : 0;
$post = get_post( $current_post_id );
?>
<div class="entry-content">
  <?php if ( $post ) : ?>
    <h1><?php echo get_the_title($post); ?></h1>
    <?php echo apply_filters( 'the_content', get_the_content($post) ); ?>
  <?php endif; ?>
</div>