He would come back in an hour if he could, but he can’t. :lol:
(My point: The sentence “He would come back in an hour” simply sounds incomplete.)
The same is true for your second example. You started out with “will” and then changed over to “would” without any reason at all and therefore “No, it wouldn’t” also sounds incomplete and wrong. You have to stick with “will” (or “won’t”) for that kind of a short response to a “will” statement.
As Mr. M. said, the uses and meanings of “will” and “would” are different. What led you to believe that they might be so easily interchangeable?