The rear spindle nut needs to be very tight, in the order of 160lbft tight. It's only the friction that the tension in the spindle creates that stops the spindle moving out of alignment. The adjuster screws and bars play no meaningful part in holding things in place against the forces generated between the drive belt and the tyre contact point; they bend really easily.if the nut isn't tight enough.
I'll stand by my description of the correct way to do up the spindle as "effing tight, then tighten till you can get the pin through the castellations."
Good approach
Those adjustment screws are clearly there to help space and angle it with the spindle loose, they’re nowhere near strong enough to support everything safely.
