Four of the five photos in the first post make me cringe with the improper use of fasteners.
Photo one, needs a flat washer installed between the frame and castle nut, allowing the cotter pin to be properly installed in the slots of the castle nut.
Photo two, Axle bolt too short, no flat washer between frame and nut. Drilling a nut could possibly weaken it and cause a failure. Use a castle nut.
Photo three, needs at least a flat washer between frame and nut. Also a lock washer or Loctite.
Photo four, axle bolt too short. At least two threads should be showing when nut tightened.
Btw, I do not mean any disrespect by pointing out these issues.
We are all here to learn from each other.