GMAT语法问题求救！！！Seldom more than 40 feet wide and 12 feet deep

GMAT语法问题求救！！！
Seldom more than 40 feet wide and 12 feet deep , but it ran 363 miles across the rugged wilderness of upstate New York, the Eric Chnal connected the Hudson River .......是OG上的题目。问：1。DEEP后有了逗号后，第一句成为了修饰句，它就一定要修饰BUT吗？不能与第二句联合起来修饰Erie Canal吗？ 2. but 有没有对称原则？还有一题：It will not be possible to implicate melting sea ice in the coastal flooding that many global warming models have projected.问：that 修饰什么？修饰coastal flooding吗？还有我刚开始看G，语法题对了8个，学一个月去考可以吗？谢谢！