Hi Boywonder and Twoxentrix
Yes there's a "stop" under the swing arm on the rear bumper. The arm doesn't move, or at most a 1mm but it seems pretty solid.
After looking at the whole system and thinking about if for a little while. It seems to me that the flat plate that is welded to the ends of the two tubes should have some pins that mate into ends of the tubes. Then the tubes will be taking some of the weight and supporting the load, rather than all of it being held together by the welds. That's how any number of things are put together to make the joints stronger. But I'm not an engineer or fabricator so I may be missing something.
I'm assuming that the local shop won't be able to weld the aluminum back together, but there's always hope. If not, I'm thinking of having them help me attach the whole wheel to the gate through the "spokes" with a couple of straps and then additionally wrapping the whole tire to the gate with a couple of extra straps N/S and E/W to reduce movement and sway on the spare. There's a hardware store in town so I expect I can get the necessary items there.
That's the best thing of can think of rather than having it inside the van and not tied down since I don't a way of securing it to the floor. That seems potentially more hazardous if I have to stop suddenly.
Thanks all for the responses.