Thanks for the response. It's Murphy's Law. As soon as I posted my question and went for a cup of coffee my reasoning and question were in jeopardy. Now it works! The last thing I tried seems to be what fixed it for me, but when I tried it before posting, I still got the "pending" status so I didn't think to wait a few seconds and try again (and now I read that I should have at least gotten "processing", not even "pending" so maybe my editor buffer hadn't been saved). I had mistakenly been sending an empty payload for the last day, not an empty structure payload. Big difference between ("") and ("{}"). When I saw your suggestion that it looked like I wasn't triggering the challenge correctly, I went and checked that I was Base64URL encoding the empty structure payload, and I was, but because I added another println to see the payload for myself, and reran, it just worked, now I get "ready" and even "valid"! And reasoning backwards, I see the last thing I did before posting was change the request payload from an empty string to "{}".
So very long winded question and follow up. Sorry. Thanks for the help!!