Jacking on the U bolt puts all of the trailer weight on a very small area of the axle tube and the axle tube may collapse from this concentrated weight.
Jacking the U bolt will not collapse a axle tube on a std leaf spring suspension nor will jacking the tube itself.
Jacking from the U bolt or the axle tube next to the U bolt on a 6k axle is lifting 2500-3000 lbs and not the whole side of the trailer when changing a flat tire on the axle.
As for the trailer aide when a tire is flat the other tire is carrying all the weight on that side of the trailer.
Another misconception.
The good tire takes very little weight because of action of the equalizer bar. Now if the trailer didn't have a equalizer bar then the good tire take some of the flat tires weight.
Same with a torsion axle without a the bar. The flat tire is still carrying a load.
Run a flat tire till the tire is gone and the steel wheel starts to roll on the road surface makes one hell of a nite time spark show. I've seen that show several times at nite when I hauled on the road for a living.