Fixed it! But still could use referral.
Between Monterey and Santa Cruz. All else being equal, I would rather drive ANYWHERE in CA, from Oregon to Baja, and meet face-to-face for (I hope) a quick fix, than ship this huge heavy thing in the middle of the best of the outdoor season to the far away unknown.
I may still need to in the future.
Hubby and I got it back on with WD40, a furniture clamp, a giant screwdriver, and a prybar. Basically the same way you put the tire on a bicycle, but no worry about tearing the inner tube. Oh, and a deadblow hammer. What we did was we seated as much of the tire as we could, which left the rest of the tire on a chord spanning about 20 degrees of the circle. (Does this make sense?) We kept it in place with the furniture clamp and starting at the opposite side of the wheel, whacked the tire with the deadblow hammer at as shallow and angle as we could. I got the idea from a youtube of a fellow who closed a gap in the rubber that was showing the wires, by striking the wheel sideways against the concrete floor. He used WD too. So we kept whacking diagonally till we got to the near end of the chord. This bought us about three inches lengthwise, which was enough to pry the tire back over the rim. Seemed impossible till we sprayed it with WD40, then it was amazingly easy. Then whacked it all back the other way, to even it out.
The interesting part was re-installng the wheel on the cart. The bearings were a little tricky but since Joe has reconditioned his Harley wheels, and I’ve serviced bicycle bearings, we eventually figured it out. (Turns out we coulda replace the tire without removing wheel.)
Delighted to be back in action, but I do think that the reason it came off is, as plain n tall said, the internal wires are stretched. That tire is still some looser than its mate. So still interested in referral.
Shared this story because I am so grateful for my amazing Joe (and proud of us), and because I thought it might offer hope to another in a similar sitch sometime.