For the work related stress person, all you need to do is keep a diary of what work assignments were issued to them on any given day, showing which ones they were happy with and able to complete and which ones triggered incidences of going off, sick.
When the evaded tasks ate reassigned to some other member of staff, get this other person to rate the work as easy/hard, rewarding/menial and so on.
With accumulating records of other staff coping easily with tasks that trigger their stress, you should be able to dismiss as unfit for duties and I doubt they would stand a chance at a tribunal (unless they'd been subjected to a bullying campaign, or similar).
I do wish employers were less scared of getting procedures wrong, more decisive and willing to take the financial hit of one or two mistakes, as they learn to get it right. Less distress for employees on the way out, too. Nothing worse than being the mouse while the cat plays with its food.