I'm kinda insider, and your description is slightly inaccurate. First, it's really long time since they've stopped using Telepath and their SMSCs are now completely in cloud
first part of your 4th paragraph is generally accurate. Non-delivered pile doesn't exist AFAIK (when message is discarded, it will be logged in the same manner as delivered message). I don't know about their deployment (they may always use homerouting) , but usually, unless the message is sent internationally, message will be delivered straight from the first SMSC.
From what I heard, Sprint did some kind of cloud migration and I guess that now they started some forgotten node. You are right that if the message is over it's validity period, it shouldn't be delivered, but discarded, but it's so basic feature that I highly doubt that it wouldn't work - I guess that either they had it set for a really long time, or didn't have properly set NTPd (if node started and thought that it was still February, it's just natural, that it would deliver it all).