Hi Yoshi, To understand the phrase by now, I suggest starting with understanding the sense of "by" that is in use. In this phrase, by indicates the end of a particular period of time that this hypothetical situation could have occurred. I wouldn't say that "now" or "by Now" are interchangeable; that said, you can say "by now" in all your provided examples because they all indicate events that could have occurred at some time in the near past. "Now" by itself just means at the present time or moment. Here are some examples of "now," "by now," and "by+ other time frame: I have to finish writing this answer by midnight. I expect the package to arrive by tomorrow. I should have turned in my essay by now, but I had a family emergency. Now is the time for action. I hope this helps!

Wenwei's explanation was very good. I will add to it by saying that using the phrase "by now" implies that the present time is the maximum expected or allowable time for something to occur.