Thanks for the response, but I already went through that checklist.
1) I copied and pasted the URL from control panel, directly into my browser to verify I typed it correctly. It loaded.
2) Even if it hit the "wrong" URL, I would still see it my access_logs.
3) We run https on port 443, which authorize.net supports.
4) The SSL is valid as all top 3 googles "ssl validation checker" reported.
5) The "test.authorize.net" which has been said a few times around these forums is quite slower / unsteady compared to the real envirnonment. If my scripts handle the test site without errors, then that elimates the variable of my scripts.
Unfortunately, this falls down to the SNI - SSL problem. Currently trying to get approved for another IPv4 address, along with sending an email to auth.net support.
I'll update this post when any details for future googlers.
If AUTH.net reads this. Why not add the same logging you have for relay_response for silent post? A simple "x failed with error when trying to POST"