If I know that certain risks cannot be prevented and are part of the process, would I still be expected to document even partial preventative measures or would I go straight for corrective steps?

It’s true that there is no way to prevent certain risks, as you say, in certain essential process steps. In those cases it’s definitely a good idea to build robust contingency plans as you suggest. There are some processes where there’s no way around the risk, but if a process hasn’t been assessed for risk…