For a September or early October journey with perhaps a week, any feedback on general preference for sticking to the north (Istria, Zagreb, Plitvice) or also trying to include Korcula, Dubrovnic, and others (if so, suggestions welcome)? Also, any thoughts on solo female travel to Montenegro and such?
No problems for a single female traveler. I covered that area in 2015.
By late September or early October you might be caught in the shift to fall weather. I guess you get to decide whether you'd rather be in Zagreb (lots of good museums) in the event of chilly rain or will hope that it's enough warmer in the south to matter. I suggest checking recent day-by-day historical weather stats for Zagreb and Dubrovnik in October on wunderground.com. Choose History > Monthly. You can look at Oct 2017, 2016, 2015 etc.
If you only have one week, you should pick one region without trying to cover most of the length of the country. The coastline is very long. If you want to see Montenegro, then focus on the south (fly into Dubrovnik or Split). Croatia is very safe and mellow, there's no issue traveling solo.