求助帖~曼哈顿语法chapter5-02

cici_gmat

两道关于paralleism的改错题：
1, Although we were sitting in the bleachers, the baseball game was as exciting to us as the people sitting behind home plate.

答案为：
Although we were sitting in the bleachers, the baseball game was as exciting to us as TO the people sitting behind home plate.

QUESTION：
为什么有的时候介词to可以省略，有的时候就不行？在这个问题里面为什么一定要加to，的确加了to读的比较顺，但是是否有比较专业的解释？

2, Tobacco companies, shaken by a string of legal setbacks in the United States, but which retain strong growth prospects in the developing world, face an uncertain future.

答案为：
Tobacco companies, which have been shaken by a string of legal setbacks in the United States, but which retain strong growth prospects in the developing world, face an uncertain future.

QUESTION：
我是否可以改为：obacco companies, shaken by a string of legal setbacks in the United States, but is retaining strong growth prospects in the developing world, face an uncertain future. 还有就是是否but前面的逗号要删除？

cici_gmat