Progress has been slow, but not non-existent:
After much measuring I decided to use the new wheels I bought.
Why much measuring? Well, it's difficult to measure but the hubs on the trailer are only about 4mm wider that the embossed rim that constitutes one of the mating surface on the new rims.
i.e. The rims are stud-centric and have pressed inner and outer rings either side of the stud circle. These constitute the mating/support surfaces.
You torque the lug nuts and these two rings make contact with the hub and that's it. The question - is the hub big enough diameter to mate with that outer ring?
In the end, I coated the inside of the rim with black marker, mounted the rims up and torqued the lug nuts to 60 ft-lb.
When I took the rim back off, I had a nice black circle on the face of the hub about 2-3 mm in from the edge. So I was convinced the rim was mounting up to the face of the hub properly.
The other thing we've done is fabricate new spring mounts. the old ones consisted of fairly light brackets welded to very thin angle ~2 mm that bolted to the trailer frame. Far too light for my taste.
We copied these in very heavy steel ~4 to 5 mm wall thickness. Should help me sleep well.
Another positive - went to dinner with the family at a local pub. My brother in law was quite upbeat.
He said, "I actually saw a teardrop camper today. NOW I get it. What a great idea."