If it was a Citroen I'd say your trailing arm bearings are knackered. I must
admit despite having owned a 406 for the last 3 years I don't really have
much idea what the rear suspension looks like. I crawl underneath twice a
year to inspect things and so far nothing has looked like it needs doing so
I tend to forget about it.

As the Haynes Manual has a section on "Rear wheel toe setting", I guess the
answer is yes, you can true them.   There is apparently a track rod which is
adjusted in similar fashion to the front.

Yes it can be adjusted, my 406 estate needs to be watched carefully as
it once destroyed a set of rear tyres in less than three months - that
gets expensive. Any reasonably tyre fitter should know how to do it.

Yes the top tie rod is on an eccentric , if ever you change them mark
the rod 1st