Reliably trapping end of paragraph / return to vertical mode

[William F Hammond]

"Taylor, P" <P.Taylor at rhul.ac.uk> writes in part:

> ...  Although Knuth provides \everypar, which can be
> reliably used to trap entry to a paragraph, there does not
> appear (to me, at least) to be any 100% reliable method of
> trapping exit therefrom, and of course once the next
> paragraph has been entered, the previous (and desired)
> value of \prevgraf has been lost.  ...

I'm going to present what is going through my mind as a list
of questions.  I am not in urgent need of answers for
myself.

1.  Does \par begin or end a paragraph?

2.  Does a blank line begin or end a paragraph?

3.  Is there any difference at all between \par
    and a blank line?

4.  For the previous questions is there any difference
    between plainTeX and LaTeX?


                              -- Bill