Thanks again for the reply.
Within few milliseconds I see the 202 from the POST. I have yet to see the GET request to my callback URL.
On sending my own artificial GET requests to the callback URL, I can of course emulate the proper fields. The problem is not receiving a GET request from twitch in the first place after the initial POST.
So to answer your question explicitly,
No, due to twitch not sending a GET request in response to the initial POST.
The GET handler of the callback URL can be fiddled with if need be when something comes from twitch.