This is correct. Depending on the stakes, the right answer would be to err on the side of caution. Certainly repeated incompetence in a private setting would be grounds for suspension or termination.

Yes and: fraud and errors are often indistinguishable.

People with malice like Elon Musk have noticed the widespread use of this aphorism and repeatedly leverage it to their advantage.

At what point does incompetence /become/ malice?

There is certainly a level of incompetence that requires active ignorance to one's naivety. I'd certainly consider a stubborn person who arrogantly ignores concerns of experts malicious. The active nature certainly matters.

Yes. Consider the concept of negligence. It is malicious to take action without exercising reasonable care, and part of reasonable care is ensuring that you are the slightest bit qualified to perform the action.

I obviously agree, but for anyone reading along, this is also the legal definition: reasonable care. Reasonable is determined by peers, not the general population. So...