The door handles maybe like the Mazak used on Morris Minors and the like which pits in much the same way! Decent replacements is the only way to go I would have thought.

The handles are made of either a polished die cast zinc (aka Zamak or Mazak) or brass alloy which is chrome plated (never stripped one down to find out) but pitting is a common issue depending on the quality (ie initial cost/quality of manufacture) of the base materials - it's a form of electrolytic corrosion. The internal chemical corrosion within the alloy 'erupts' and breaks the thin chrome coating hence the pitting, once the coating integrity is broken moisture gets in and it slowly spreads/deteriorates. Easiest fix is to try and get hold of a more 'perfect' set or refurbish as suggested by spraying, re-chroming is possible but expensive and it's probably going to do the same again in maybe 5-10 years time if your lucky. I think they are still available new but quite expensive.
The stainless steel rear window trim has a black rubber/plastic trim bonded onto each edge and it's that trim that is breaking down/gone brittle. Members have disguised this effect by clever masking and careful spaying with black rubberised or vinyl products (you'll find a thread somewhere on here). I managed to obtain a good used replacement off a member on here or you can still obtain new from Rimmers if you have +£130 spare...