If your mechanic says 'Timing chains are for life and require no maintenance' then please find a new mechanic.
There are loads of examples of timing chains that are designed for life but suffer from early failure of some sort whether its stretching, snapping, a tensioner or guide failing or anything else.
BMW N47 2.0D - Probably the most well known, simple bad design mean they snap with no warning as low as 60k.
Ford 2.2, 2.4 TDCI - They stretch, sometimes you get warning with a rattle noise, sometimes not and the chain jumps causing massive engine damage.
Mitsubishi 3.2Di-DC 4M41 - The top chain guide breaks causing a loose chain and in some cases the chain jumps.
The list goes on including VW examples that I will search for the engine codes and update my answer.
As for preventative maintenance then yes this can prevent a failure, depending any known issues then it could be as simple as taking the rocker cover off and replacing as in the above Mitsubishi case or in the BMW the engine has to be removed, sump taken off and the chain can then be replaced. In many cases there will be a upgraded kit available in the aftermarket or the manufacturer will have superseded stronger parts.
Question 1: No not all many engines especially diesels have belts.
Question 2: Yes. If the manufacture says it the chain is for life don't take a bit of notice, do your research, look through online forums, speak to mechanics and look at repair info sources such as Autodata for known faults and bulletins.
You may find that the majority of people have hundreds of thousands of miles with no issues or that the chain seems to be failing very soon. Depending on what you find it may be as simple as changing a guide or changing the chain itself which can be as simple as removing a timing cover and setting the timing or may involve removing as dismantling most of the engine.