Disclaimer : Please excuse me for the delayed attempt for these questions
I have attempted "table space usage" questions 1,2 and 3 in sequence, but the answer for question 3 is mentioned in discussion of question 2 :-).
I think that quote in the discussion on question 2 is about the lack of practical application. And that is true. In a real database, I want to have a good estimate about the data size I can expect, but I truly am not bothered by just a few pages more or less. However, the table designs I used in question 1 and 2 were such that an exact calculation is at least possible.
For question 3, the table design is such that based on the given information it is impossible to do this exact computation - even if you would want to.