How to change post template

The way I see it you have two options here:

1. Pass the id to a custom template and then create a custom wp_query loop and use $_GET['id'] as the id of the loop.

For Example:

<?php
/*
Template Name: Pop Up View
*/
//get id from your url 
$id=$_GET['id'];

//start custom loop

$query = new WP_Query( 'p=$id' );
//loop stuff and page template here
?>

then you would call <a href="https://wordpress.stackexchange.com/questions/76322/custom-item-template?id=<?php the_ID();?>">Link</a>

Or going more with your current code you could do something like

    if ($_GET['custom-template'] == 1) { 
       get_template_part( 'custom-template');
      die(); //or exit(); 
}

This fixed a few problems I see with your current code. One being that return can’t be used outside of functions. Also uses get_template_part instead of include with is a more WordPress way to do it.

Both ways should work, hopefully that gets you on the right track.