Answer:
Given that you have 5 major questions in one, I will answer each with a brief explanation, because this is no short subject.
1. In the human body, there are many example of two system working at the same time, or in synchrony, to perform a specific function. One such example is the respiratory and cardivascular system. Without those two working together, there would be no life, period. While the cardiovascular system pumps and transports blood to the cells of the body, the respiratory system works to provide this blood with the necessary oxygen, and to take away gas waste (CO2), so that tissues may survive.
2. Autopsies will always be required in cases when cause of death is unclear, or when foul play may be suspected. This happens in sudden deaths, unexpected or unexplained deaths, or in automobile accidents when it is unclear if it was the accident itself, or something else, that caused the death of the person. Because it is necessary to ascertain whether the cause of death was a natural one, or if there was a disease that caused it, or the act of another human being, autopsies are ordered.
3. In the case of the death of Anna Garcia, it would be important to have the following info: pre-existing medical conditions, as these will shed light on her case, also what it was that she injected on her thigh because it might explain if there was drug reaction, or foul play, if there is an explanation for her back pain consult and a full autopsy report, medical records and all the findings on her body.
4. The reason for this is that in unexpected deaths, and even in accidents, sometimes the suspected "cause of death" may not be accurate. There may be an even deeper underlying cause and therefore it must be ascertained. So, given that the unknown needs to be totally clarified, all body systems are checked to establish an almost 100% accuracy to the cause of death. This will also shed light on whether the death of the person was due to natural causes, or a disease the person had, or if there was a human hand behind the situation.
5. One problem with determining exactly a cause of death lies with the fact that there may be conditions of a person that were unknown and hidden, like an unknown disease, that may explain, or completely change a known "cause of death". Another issue is when a person has been dead for a while and the body has decomposed. The effects of decomposition, and also the exposure to elements of the body, will alter the accuracy of the findings done. Finally, sometimes there may be no absolutely definitive findings on a cause of death, no direct explanation as to why a person passed away, no traceable markings and no medical conditions either. So establishing cause of death may be impossible.