Wonder if they could claim compensation for work injury regarding their back problems.
I don’t believe so.
They can take hiatuses and sometimes fans will harass them for taking too long to create more chapters. Naoko’s husband was harassed by fans and at one point he just asked people to leave him alone.
Being a mangaka, as an insider put it, is a form of voluntary enslavement; mangakas create manga because they ultimately love manga. I don’t believe they do it for the money because there are easier and better ways to build wealth. Being a successful author and building wealth is difficult. And even if you are lucky to build wealth as a mangaka, fans will always demand more content.
Look at Takeuchi, for example. Takeuchi successfully concluded
Sailor Moon and
Sailor V, she openly admitted her struggles while crafting the story, and yet fans continue demanding more content from her. Fans always want more. It is never “good enough.”