向各位高手请教一下OG12-5

Diabetes, together with its serious complications, ranks as the nation's third leading cause of death, surpassed only by heart disease and cancer.
我有两个问题：
1、这个地方的ranks为什么不用成is ranked,我理解为应该是“被列为”？
2、OG中对“together with its serious complications”的解释是：the phrase following diabetes is set off by a pair of commas, indicating that it is off by a pair of commas, indicating that it is descriptive information that may be dropped from the sentence, it is not a part of the subject.以后我怎样可以判断它属于，还是不属于主语？
菜鸟一只，各位见笑了，望指教：）