If you take any complex system out of the environment it evolved to survive in, it won't work as well. Doesn't matter which knob you tweak, it won't be the environment it evolved in. There will just be a different set of failures.

Is this necessarily true? I can see how it would often be true, in general, but don't see how it's necessary. For example, animals in captivity can often live longer. Lifespan and quality of life are decent enough proxies for "working well".

I don't think it's absolutely true, the general case you referred to is as strong as I intended the assertion to be.

I would assume that stronger gravity would just come along with a different set of health issues (joint and muscle problems dealing with more stress, heart would be affected having to work harder to pump blood, etc).