I split the idea of a minimal, lightweight chassis into three steps:
1) If the body is going to be self-supporting, it needs to be well made. This doesn't mean heavy - it mostly means good quality joints that fit well and are glued and screwed or pinned.
2) Getting the axle load into the body is easy - all that's needed is to spread the point load from the axle over a decent length of wood. In the Ultralight PDF, this is done with the spring mount angles about 18" long that the HF trailer comes with. On my Ultralight chassis, the angles are 24" long.
3) Getting the hitch/tongue load into the trailer is a bit harder as the load is well outside the body, so it creates quite a bending moment in the tongue. For this I think the easiest solution is the Ultralight chassis that uses an angle A-frame where the back end of the A-frame members bolt to the front of the axle angles.
If you really want a single central tongue, then you need a body structure that's quite a bit more complex as it has to take the tongue loads in the middle of the floor. This needs reinforcing beams running across the floor which would ideally be a sandwich structure - for sure, just a single-skin unreinforced plywood floor is not enough. I reckon an oak 1x4 inside the floor sandwich at the front and back ends of the tongue would be enough, providing the tongue length inside the body is not much less than the tongue length forward of the body. The backing plates at the tongue fixing points are absolutely essential, otherwise the bolts will just pull out of the floor.
But this is getting sufficiently complex that it's less work to use the stronger A-frame chassis.
Andrew