croatia in may

1. I didn't go to Mostar, so can't help you there. 2. Kotor is a good day trip. We rented a car for the day (started early) and enjoyed a full day - it worked out perfectly. 3. I don't think you will be able to do Korcula as a day trip. We had one full day and 2 nights there, and that was about right. Getting to/from Korcula from Dubrovnik is not difficult but takes at least a few hours each way. You'll want to spend the night anyway. 4. All the recommendations you could ever want (and more) are in Rick's book. You do have it, right? Having the book, we never felt any need for a guide. 5. I would think it's perfectly safe for any solo traveler who uses common sense, like anywhere else. 6. One full day and 2 nights in Dubrovnik was all I needed/wanted - it's worth seeing, but I wouldn't linger there. It will be crowded. 7. Weather in early May can vary widely - be prepared for everything from cold and rainy to warm and sunny. You won't be doing any swimming.
We were there for about 3 weeks in May 2012 - amazing area.

We didn't visit Mostar either, but took a bus tour to Bay of Kotor and it was very enjoyable. I agree that Korcula would be best done as an overnight. And finally, rather than staying in a sobe in Dubrovnik, you might look at B&B's in Cavtat. This is a pretty little fishing village south of Dubrovnik, near the airport. You can reach Dubrovnik in about 10 minutes by bus, or 40 minutes by more scenic boat. You can see the Dubrovnik harbor from Cavtat, and time your visit for a day when it's not overrun by cruise ships. We did the Kotor trip from Cavtat, and am sure you could take a Mostar tour from here as well. Check Trip Advisor under Dubrovnik Activities for tour guide recommendations.

Yes, no problems traveling alone (I traveled with a friend but would have no problem alone). We took two one-day trips booked through Viator - one to Montanegro (including Kotor and other sites) and one to Korcula on two separate days. The Korcula trip was the better of the two - it felt much more relaxed...these were long days but worth it! We came to Dubrovnik via Mostar by train and bus (through Ploce), but I think you can just take a bus. I'm not sure about sobes because I'm (just guessing, not sure) that they may not be advertised as readily on the internet - they are rooms rented out by owners (who show up at ferry stops to meet new arrivals and entice them to board with them). Try looking at B & Bs or small lodgings for Dubrovnik on booking.com (or other Dubrovnik apartment websites or Rick Steves book)