2 words, very similar in meaning, however different in definition. They can be swapped but only at certain locations. Critical can refer to an urgent matter as would important. But something that's important may not necessarily be critical. So, it depends on the situation in my opinion. I'm sure others here might have a different idea.

Yes. It can mean "vitally important". Example: "It is critical that you come to the hospital right now." Meaning, "It is vitally important that you come to the hospital right now." Another example: "Your sister is in critical condition." Meaning that she is not in good condition. Something is terribly wrong; it is critical.