Let me tell you what worked for me.
I unpaired and re-paired my Apple Watch to my iPhone. I used my S2 info for a restore... it doesn't seem to care if you set up new or from restore.
Once I had re-paired it (this was my 3rd re-pairing, btw) and both the phone and the watch were showing as all set up as usual, I then power-cycled my watch AND my phone. This power-cycling of the phone was the very first time I'd power-cycled my phone the whole time I had this ordeal going on.
I cannot tell you what sort of voodoo happened, but when the phone and watch were both powered back on, I could then go into the Watch app under "Cellular" and the option to Set Up Cellular was no longer grayed out.
Granted, it is still in "Activating" mode and might just not work, but I managed to get this far and it's farther than I got the last few times, so there's that.
Ugh. This reminds me of when I bought the original iPhone on launch day and it took 3 days to finally activate. Very poor coordination of this launch with the carriers.
ETA: It worked! Is all set up now. Also, during NumberSync process it did show that there was a 55-dollar credit (25 to waive activation and 30 for 3 months of service) that I would get over 3 months. It did not say which 3 months, though. LOL.