"The man was elderly and infirm. We can understand his taking an evening stroll， but the ground was damp and the night inclement. 

Is it natural that he should stand for five or ten minutes， as Dr. Mortimer， with more practical sense than I should have given him credit for， deduced from the cigar ash？"

上面不理解为什么使用should ？是什么用法和意思？

（指Is it natural that he should stand... 和...with more practical sense than I should have given him credit for)

1)natural是含有感情的用词吗？类似于sorry,regret, surprising， 如果是的话，这个我就能理解了。翻译为“居然，竟然”，含有感情之意，当然

也可以使用陈述语气一般过去时stood（无感情之意），我的理解对吗？

2) 但是第二个should不能理解：with more practical sense than I should have given him credit for.

是不是这里是should have done - 

a.  与过去不符的虚拟语气

b.  对过去情况的可能性推测

我认为应该是b. 对过去情况的可能性推测，对吗？

这里的should可以换用成would吗？即with more practical sense than I “would" have given him credit for.