Can I simplify in function column_default use of queries

Your problem is mostly just a change in PHP logic, not really WordPress. Anyway, we are here to help, and that would be off-topic on other sites too, so …

1. It is unfortunate enough that you have to access a global variable in order to use the current database connection. Make others (you in 6 months!) life easier, and import global variables always at the top of a function body.

   If you are searching later for all accesses to that variables, you will thank your past self for that. 🙂
   In my code, I use mostly class methods as callbacks, and I pass the $wpdb object as a dependency into the class constructor, so I have to touch that global variable just once, at the very top of my code hierarchy.

2. When you are using almost the same query every time, look what the differences are, and make those either a variable, or unify them. In your case, you can just use the $column_name variable, because it is matching the database table column name and a whitelist of valid strings. Also, you can rename the resulting variable: SELECT x as foo FROM …

3. $item[ 'ID' ] is unknown, it can be anything. Always prepare unknown values before you send them to the database. $wpdb has method prepare() that works similar to sprintf().

4. You don’t need to run count() on the query result. You get an object only on success, so the faster test against is_object() is good enough.

5. Move that code to a separate function. It does too much.

Sample code for all of these points:

function answer_query( $column_name, $id )
{
    global $wpdb;

    $sql = "SELECT %s as result
            FROM {$wpdb->prefix}rdp_participants_answers
            WHERE user_id = %d ORDER BY id DESC LIMIT 1 ";

    $query = $wpdb->get_row( $wpdb->prepare( $sql, $column_name, $id ) );

    if ( is_object( $query ) )
        return $query->result;

    return 0;
}

And in your switch statement, shorten the code to:

case 'answer':
case 'answer_time':
case 'ip_address':
    return answer_query( $column_name, (int) $item['ID'] ) );