请教比较结构中的谓语动词省略问题

〉〉白勇书上说从句谓语与主句谓语相同，可用do，did或does之类代替。但是除非主从句主谓都相同，一般不能省。

参考白勇第7章29题解释：

29．Local residents claim that San Antonio, Texas, has more good Mexican American restaurants than any city does in the United States.

(A)   any city does

(B)   does any other city

(C)   other cities do

(D)  any city

(E)   other cities

答案是B很明显，可是他对D，E项错误的解释是：缺少do/does 代替重复的动词has.

这里似乎表明：重复的动词不能省略。

〉〉但是OG中就有反例，比如：OG11REVIEW第75题

75．Salt deposits and moisture threaten to destroy the Mohenjo-Daro excavation in Pakistan, the site of an ancient civilization that flourished at the same time as the civilizations in the Nile delta and the river valleys of the Tigris and Euphrates.

(A) that flourished at the same time as the civilizations

(B) that had flourished at the same time as had the civilizations

(C) that flourished at the same time those had

(D) flourishing at the same time as those did

(F)   flourishing at the same time as those were

答案A中并没有补出动词did.

我的问题是：

（1）       什么情况下比较结构中从句的谓语动词可省？

（2）       若补出谓语，它的位置应在名词前形成倒装，还是放在名词后?是否看其后跟的名词的长度，若太长，就倒装？

（3）    用do来代替原来的动词在GMAT中是必须的还是可选的？

查了许多以前的帖子，对这个问题还是不能理解地很清晰，

请各位NN指教。