After hours of searching, scouring, absorbing it was determined that there was little hope of (currently) getting past Services 2.4 and REST returning a 404 error, so I switched to xmlrpc.

I have a simple script to hit my service from outside the site, fine-tuned a couple dozen times based on various recommendations, to currently be:

$url = "http://localhost/mydomain/services/xmlrpc";
$ch = curl_init();
$data = 'method=my.method&id=12345&text=hello';
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_URL, $url);

$response = curl_exec($ch);

with $data as a string like above, I continuously receive: faultCode  -32700    faultString  Parse error. Request not well formed. 

If I change the postfields to be an array('method'=>'my.method','id'=>12345...  I instead continuously receive a message that xmlrpc will only handle Post transactions.