So my first question is how many hook the truck to the fifth wheel while changing tires and why?
I don't hook the truck up to a trailer just to work on the trailers axles/tires/brakes/etc. Need the truck to ferry the wheels to the tire dealer. I leave the trailer on the landing jacks and simply raise the end of each axle next to the U bolt with my floor jacks. As was mentioned I'm only raising the axle(s) maybe a 1/2" for the tire to come off the ground.
Second question, my last fiver I used a large bottle Jack under the leaf spring between the u-bolts and jacked just till the fire raised off the ground. I know most manufac. say Jack by the frame which is kinda high. I could easily make it work by cutting some 2x6's and screwing them together and set the bottle Jack on that???
Your scenario works fine. My 12 ton shorty hyd jack has a short leg channel that fits over the end of the jack which cradle the axle tube. Eliminates the axle from slipping off the jack while on soft ground especially when on the side of the interstate with trucks blasting by at 75 mph.
Think safety....once the wheel is removed use some type of cribing or use a solid jack in case the hyd jack lets go.