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.