What does the classical construction X 何由 Y mean?
in classic Chinese, 何由 is the inverse form of 由何, and it should be in the inverse form ,however, you can understand it by understanding 由何.

何 what
由 [preposition] 1. from  2.because of  3.by(method, means,way)

so 由何 could mean, from what, because of what, by what. I guess, you would less commonly(Ok, at least I would) say,&quot;By what, did you arrive at this conclusion?&quot;, instead, you would say,&quot;how did you arrive at this conclusion?&quot; ;furthermore, when you say &quot;how did you arrive at this conclusion?&quot;, you would probably mean &quot;It's impossible to arrive at this conclusion. I don't believe it&quot;, depending on the context. This also happens to 何由.
In your examples,I think 何由 means &quot;how could&quot;.
By the way, I think this word is not rarely used in classic Chinese, I can give you another example from a poem 《茅屋为秋风所破歌》 by the great poet 杜甫, which was picked into the textbook when I was a student.

长夜沾湿何由彻  How could I bear the bad situation(the house is broken and it rains heavily) in the long night , until the dawn comes.