Balkans itinerary - is it realistic??

I would prefer more time in fewer countries, but I suppose you can pull this off. Just be aware that border-crossing trains and buses are typically infrequent; if something goes wrong with one of your transportation legs, the whole trip is going to come crashing down.

You are rushing through areas that have much more to offer. Just among the places I've been:

Serbia: Novi Sad (not far from Beograd)
Montenegro: Ulcinj, Cetinje, Herceg-Novi, Budva (though rather similar to Kotor)
Croatia: Split, Zadar, Plitvice Lakes National Park, Zagreb, the islands, the Istrian Peninsula

Don't wait till the day of travel to buy bus tickets (or train tickets if any require seat reservations). I have occasionally seen buses sell out, though it is uncommon.

It's quite a rushed trip. Plus, it's hard to enjoy a night out if you have to catch a bus in the morning. If you have to catch a flight from Dubrovnik, consider basing yourself there instead of in Kotor and then visiting the bay as a day trip from Dubrovnik. The morning buses from Mostar to Kotor go through Dubrovnik anyway.

We did almost that same trip 3 years ago. We started in Budapest as well - how very funny. We flew from Budapest to Serbia, having heard some dodgy stories about the train - those stories are now 10 years old. I used a guide from the Beograd tourist office named Djani (pronounced "Johnnie") in the office close to the statue of King Michael. He drove us to Sarajevo for a pretty competitive rate, plus we did not use the airport and all that hastle. If you are solo, probably expensive. For 2 or especially 3, it would be good. I think the itinerary is rushed. I would go from Mostar to Dubrovnik and stay there. Kotor is quite like Dubrovnik, just smaller and less interesting. A little less touristy, true. In Dubrovnik, make sure to visit our favorite bar, Buzz Bar. It's off the main street, and reasonable. In Beograd, there are the river boat restaurants (we were there in late October and all were closed). In Sarajevo, we enjoyed the "Time of Troubles" tour and a beer at the famous Sarajevo brewery. We went from Sarajevo to Dubrovnik by private car, which again was reasonable for 2 or 3 but possibly expensive for 1.

I did a similar itinerary but we started in Romania and flew out of Zadar, so this was the middle of our itinerary We spent more time. First check and make sure that train is the cheapest/best option - flying might also be.

We took a bus from Belgrade to Sarajevo. It was fine. Then we did a bus to Mostar, but we stayed overnight. We wanted to go to Kotor but skipped it because it was just too time consuming to get there. I would recommend doing the same, honestly.

We actually took a taxi from Mostar to Dubrovnik - sounds silly, but the driver offered us a good price and we'd been doing tons of bus travel and it was nice to have a bit of luxury. For Dubrovnik I am really glad we stayed in the nearby town of Cavat. It was beautiful and restful, yet it's just a quick boat or bus ride to Dubrovnik proper to rub shoulders with the crowds.