I just tried out longlines-mode with AUCTeX/preview-latex.  Cough,
cough.  One problem that it has is that it does not preserve markers
around spaces/newlines.

Emacs' fill modes have got this right by now, in contrast.  The main
trick is that if one replaces one kind of space with another one, that
one _first_ adds the new space, then deletes the old space.  Here is
how to do this in one case (unless I am mistaken):