template_include for search.php makes Wordpress think its on the home page

I have a custom tag-search feature, which allows people to search multiple tags to return relevant posts. Say they want to see posts that are tagged both 'Wordpress' and 'Tutorials'. I'm using the template_include filter, like so:

public static function template_redirect( $template ) {
    if( isset($_GET['tag-search']) )
    {
        $template = locate_template( array(
            'search.php',
            'archive.php',
            'index.php'
        ));
    }
    return $template;
}

the tag-search variable holds the name of the taxonomy, I check if it is set, thus indicating a search is being performed.

At this point template resolves to search.php, like it should. So that's good.

In the pre_get_posts action I add a tax_query to modify the returned posts to just the selected tags, and I set is_search to true, and is_home to false. Here is my pre_get_posts action:

public static function pre_get_posts($query) {
    if( !isset($_GET['tag-search']) || !isset($_GET['terms'])  ) return $query;

    if( strtolower( $_GET['method'] ) == 'and' )
    {
        $query->set( 'tax_query', array(
            array(
                'taxonomy' => $_GET['tag-search'],
                'field' => 'slug',
                'terms' => explode(',', $_GET['terms']),
                'operator' => 'AND'
            )
        ) );
    }
    else
    {
        $query->set( 'tax_query', array(
            array(
                'taxonomy' => $_GET['tag-search'],
                'field' => 'slug',
                'terms' => explode(',', $_GET['terms'])
            )
        ) );
    }
    $query->is_search = true;
    $query->is_home = false;
    return $query;
}

However, when I perform the search, the body class is 'home', and all of the is_home and is_front_page conditionals are true, which they shouldn't be, cause I just specifically stated I want this to be a search template right?

I know this can be done - when I add &s to the query string it works as it should - though it's a bit ugly that way. Any insights?