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I wonder if the lower control arm ball joint cup needs to welded in at an angle........?
Not until you really figure out what is going on!
There may be a lot going on here, please excuse me if I mention some obvious things, but some people reading this might benefit from that. I did some FEA for a member here and we found the lower rear arm was flexing a great deal. I don't remember the number but it was well over an inch. At that he said that this explained why the jam nuts on his rod ends refused to stay tight. The rod ends couldn't come loose because of how they were bolted on, but never the less, the jam nuts always backed out. When you don't use trailing arms there can be a lot of torque on the arms from braking and the offset of the coilover mount. So I advise the trailing arms, they make for a simpler set up that is easier to understand. I know that doesn't help you now though...
Another thing is that the spindle, axle pin and rim also flex, none of which is helping you.
For the FEA, I would set the balljoint fixed vertically but allow rotation and movement horizontally. The inner pivots should be fixed vertically and horizontally but allow rotation. The FEA is showing you the stress in that picture but not the strain, I assume. So the flat plate may not be stressed but allowing a lot of bending at the same time. Welding the tubes to the ball joint collar will help with that.
On my car's lower arm I found significant stiffness with a tube crossing about where the back of the plate is. I'll try to find my pictures of this.
Your spindle may also be rotating under braking, that would make it look worse on one side compared to the other side of the car.