I wonder if I could open the document and remember the last scroll position so next time I reopen it, it straight nagivgates to where it was.

You may find THIS to be of interest on that topic. There’s a Pythonscript there that supposedly solves this “problem”. I haven’t tried the script out yet, so I can’t fully endorse it. It’s on my evergrowing “TODO” list. :-)

remember the last scroll position

if you close/exit Notepad++ and reload the application, it does remember the scroll position for the active session. But no, once you close the document, it loses that scroll information.

([a-z,.?! ][,.?!])(?=[a-z])

it’s not perfect, but it does the job anyway :)
only downside is it matches preceding characters as well, so it looks less charming.

Any regex that uses \K match-resets or (?<=...) / (?<!...) lookbehinds require using replace all rather than single replace actions. That’s the way it was designed, to avoid certain …confusions… that would arise with reset- and lookbehind- replacements run one-at-a-time.