The placement of the differential mounting holes in the underside of the tube, removing a substantial amount of material from the piece of the design that's intended to supply the strength is suspect. Also once located there, a single pass weld is most definitely insufficient. Even with excellent material, conditions, and skill I believe that failures like that will be common. At the very least the main tube needed to moved several mm lower so the penetration for the sleeves that his passed directly through the center of the cross member and a multiple pass weld with attention paid to temperature during the welding process and probably a post treatment like normalization wouldn't be out of the question.
Fixing a pre installed unit prior to failure will be difficult to do discreetly, with the material used. A better design would have a plate welded at the center of 2 oval shaped tube crossbmembers ( not circle)with a short but long gusset running nearly the entire length of the bar perpendicular to the long side of the oval in order to prevent folding it in half. There are other small changes I would but not wanting to type them.
"I couldn't find the sports car of my dreams, so I built it Myself." -Ferdinand Porsche