根据来提顺序,答复如下：
1） 先肯定这里的 that 引出的是实际主语.注意,It has been estimated that 据估算 .,其后 if the bridge were packed with cars, it would still only be carrying a third of its total capacity 是一句虚拟语气,然而,这一虚拟与事实比较接近.这里的 be carrying 用得比较特殊,是有这样的用法.
2） They support the cables from which the bridge has been suspended. 这里不能孤立地看这一句句子,应当与前文同时看 “Above the surface of the water, the tower rise to a height of nearly 700 feet.” 与此事实相同,当然也要用一般现在时态,但大桥已经作为一座完工项目,作为时态呼应,此定语从句就应当用完成时态.