I have a quite a few thoughts for the OP, based on my trip a year ago:

- A couple days in Dubrovnik was enough for us. It was our last stop, and it was so overrun with tourists that after a fantastic trip, we actually couldn't wait to leave after a couple days. However, DO find the cliff bars on the outer wall overlooking the sea, and have an over-priced drink there. That was our favorite part of the city.

- Spend as much time on the islands as you can. They are more laid back, even in the touristy areas, and there are some beautiful beaches to take your 2-yr old. We found at the beaches we visited on Korcula, we could rent 2 lounge chairs with umbrella for a day for about $20 US, and there was typically some type of food available for sale. Korcula would be an excellent option as it's not too far from DBV. All of the islands are well-served by big high speed ferries.

- Use AirBnb or HomeAway if you're comfortable with that. We found better deals and really enjoyed having a little apartment. On some of the islands in particular, hotels are not a better option.

- While parts of Montenegro are absolutely gorgeous, we were frustrated with the traffic, and it's not just at the border. We had a day trip planned for about 10 hours, that lasted around 16, without us even getting to see everything we wanted, due to traffic. At one point, we were stuck in a long tunnel for nearly 2 hours, barely moving. I won't say don't go, but also don't say you weren't warned...

- We also did a bus tour to Mostar (from Split). It was a fun to see, and a good trip, but it's a little touristy. There's a park, and I'm going to mess up the spelling, I think it's Kravice Falls, in B&H that was a stop on our trip. Really beautiful set of falls you can swim in, but again, there will be a lot of people there on a nice summer day.

- With 8 nights, here's a suggestion:
2 nights in Dubrovnik. Walk the wall, see the sights, etc.
Take the ferry to Korcula. 3 nights. Explore old town, eat some fantastic seafood. Visit the beaches at Lumbarda and Pupnatska Luka. Visit one or two wineries between the Old Town and Lumbarda. Try some wines, but the setting and the views are amazing.
One other island (Hvar might be a good choice) for 2 nights.
Back to DBV for the final night.

We LOVED Korcula. Hvar not as much, but there's a lot of infrastructure there. But lots of islands to choose from. One that intrigued me when we stopped by for a couple hours was Vis, but not sure how it would be to stay there. Also, not an island, but right across from Korcula is Orebic, on a wine-producing peninsula. We didn't ferry over there, but were ready to if Korcula hadn't kept us busy.

If 3 different check-ins are too much for you, I'd probably consider splitting time between DBV and one of the larger islands like Korcula or Hvar, and then doing some day trips to see the other islands -- they're all a little different.