haveing just changed rear donut bushes on mine, i would suggest adding these to you list of investigations. Although i fitted carlton/senator bushes which are firmer, i can report a marked improvement in handeling/tram lineing and far less correction required at the wheel.
I have not experienced the level of deviation your post describes but sounds to me like an extreme case of the usual Omega handling problems. As a tester you have an advantage,and the usual recommendation is to examin the car for,
Front wishbone bushes, wear, tears
Stearing idler, play
Broken springs, difficult to spot.
Rectify all issues. Only then should you take the car for a Full Geometry set up as any subsequent changeing of parts will need another set up. Wheels in motion or similar do a goodjob. See also the oof map for other recommended companys near you. You need an operator who knows the Omega set up as the factory settings are so wide as to be almost useless.
Tracking on its own from most usual tire places is simply a waist of time and money as this is only a fraction of the problem.
In addition, you may also want to check,
Track rod ends.
Wishbone knuckles/king pin.
Stearing box(a nut on this to adjust play, dont over tighten)
Rear bushes outer(donut)and inner.
Feel a bit like teaching you to suck eggs but there you go. Hth