Threading can work, DB connection unterminated:

I'm not having the same issue that you are when I switch over $pid -

<?php

require('app/Mage.php');
Mage::app();


$pid = pcntl_fork();

switch($pid) {
    case -1:
        print "Could not fork!\n";
        exit;
    case 0:
        echo "In Child " . Mage::getModel('catalog/product')->getCollection()->getLastItem()->getId() . PHP_EOL;
        break;
    default:
        echo "In Parent " . Mage::getModel('catalog/product')->getCollection()->getFirstItem()->getId() . PHP_EOL;
}

Returns:

> php test.php
In Parent 1
In Child 32

However if I do not, I get a MySQL gone away error:

PHP Fatal error:  Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2013 Lost connection to MySQL server during query

Summary:

I think you have an error in your execution code somewhere wherein you are not executing database queries from within the switch over process ID.

Edit

Further investigation:

By declaring a Mage::registry value, and then inspecting the source of the query we can, from within lib/Varien/Db/Adapter/Pdo/Mysql.php, inspect the scope (pid) of the current running process:

test.php:

$pid = pcntl_fork();
Mage::register('pcntl',$pid);

lib/Varien/Db/Adapter/Pdo/Mysql.php:

public function query($sql, $bind = array())
{
    $pid = Mage::registry('pcntl');
    switch($pid){
        case 0:
            echo "In child" . PHP_EOL;
        case 1:
            echo "In parent!" . PHP_EOL;
    }
    //.... stuff

Result:

In child
In parent!
In child
In parent!
In child
In parent!
In child
In parent!
In child
In parent!
In child
In parent!
In child
In parent!
In child
In parent!
In child
In parent!
In child
In parent!

Magento 2 use InnoDb Fulltext Indexes and CURRENT_TIMESTAMP default value that not available in Mysql 5.5. So you cannot install it and skip detection step cannot help you.