Brilliantly designed!

Are the wheel axles just threaded directly into the aluminum extrusion, or is there a nut inside?

If you have any problems with anything yielding, inline skate wheels, being much thinner, will bring the wheel centerline in a great deal, to reduce axle bending moment, and they also have superb bearings. There is also always the possibility of using a long single thru-axle for both wheels, so the axle support extrusion is only loaded in vertical shear, but that adds weight and bulk when being transported.