Axle reinforcement

Why do you think the use of A36 steel for the frame is OK but not for the axle tube? Its a tube that is rigidly bolted to the frame, so you can consider it to be part of the frame structurally, it might as well be welded as part of the frame assembly. Its not like a spring axle which is directly taking the bump loads from the wheels and transmitting them to the springs, which in turn reduce the loads being transmitted to the frame. The torsion arms and torsion bars do that by transmitting those loads to the rubber cords, and the cords are the equivalent of the springs in reducing the bump loads being applied to the axle tube. 

So, its not like the axle tube "has" to be made of a particular material. It has to do a specific job in terms of its deflection and loading capabilities and a good design will accomplish that (and no more) at the lowest cost. They don't pay design engineers to over design stuff. I've run through the deflection and loading numbers on the tube and its not that bad. In our case the tube has wound up being somewhat shy of what it needs to be in the rPods because of the inboard mounting points.  if I was designing it I would be using A36 steel as well, just would have gone with the next higher wall thickness. Adding the angle under the axle tube is the equivalent of increasing the wall thickness.

So if you're really worried that adding reinforcement to the existing tube somehow makes things worse and you still want to upgrade the axle then I'd suggest going with a 5200 lb axle. That has a lot of other advantages in terms of a higher rated spindle, torsion arm, rubber cords, brakes, bearings, etc. I'd be interested in knowing what that axle tube size and wall thickness is for comparison.  

Any differences in trailer axle load should be pretty small between different wdh's assuming the same TV and trailer loads and that you've got them set up the same way, bringing the load on the TV front axle back to where it was before connecting the trailer. To estimate how much load a wdh is transferring to the TV front and trailer axles you can use this calculator. Once you have your weights and dimensions set up you can adjust the wdh tension so that the front axle load is the same with the trailer as it is with the trailer weight set to 1 lb (the calculator blows up if you set the trailer weight to zero). For me it adds about 150 lbs. Not insignificant but also not a huge increase.