Since the Fiesta and the FiST have a twist beam rear suspension:
Take out the twist beam from ^^^ that ^^^ and then explain to me how toe and camber are controlled.
If you say "its the bushing" you get an F in remedial suspension geometry class.
In traditional suspension analysis, the arms are broken down in to links, so when you hear about a "five link rear suspension", each side has five links but you can also count an A-arm as two links making two A-arms and a toe link a "five link rear suspension".
In a trailing arm independent rear suspension the trailing arm is always a two link arm. That constrains the motion in to an arc contained in a set plane.
A single link only constrains motion to a spherical surface in a 3D space rather than a plane. In layman's terms, it flops around wherever it wants.
When a twist beam axle still has its twist beam, you can ignore one side and call the twist beam an A-arm with motion constrained to an arc in a plane perpendicular to the line drawn between the two bushings. That's the typical very simplified analysis of a twist beam axle, you ignore half and leave out torsion entirely. If you cut out the connection to the other bushing, now you only have one link, no A-arm and it flops around wherever it wants.
So what does that tell you?
Don't cut out the twist beam is a good start.